Latest I2C fix raised some warnings

[fpistm]

Since #306:

hardware/STM32/stm32/cores/arduino/stm32/twi.c: In function 'HAL_I2C_AddrCallback':
hardware/STM32/stm32/cores/arduino/stm32/twi.c:551:50: warning: passing argument 2 of 'HAL_I2C_Slave_Sequential_Transmit_IT' discards 'volatile' qualifier from pointer target type [-Wdiscarded-qualifiers]
       HAL_I2C_Slave_Sequential_Transmit_IT(hi2c, obj->i2cTxRxBuffer,
                                                  ^~~
In file included from hardware/STM32/stm32/variants/DISCO_F746NG/stm32f7xx_hal_conf.h:330:0,
                 from hardware/STM32/stm32/system/Drivers/STM32F7xx_HAL_Driver/Inc/stm32f7xx_hal.h:46,
                 from hardware/STM32/stm32/system/Drivers/CMSIS/Device/ST/STM32F7xx/Include/stm32f7xx.h:219,
                 from hardware/STM32/stm32/cores/arduino/stm32/stm32_def.h:18,
                 from hardware/STM32/stm32/cores/arduino/stm32/twi.c:51:
hardware/STM32/stm32/system/Drivers/STM32F7xx_HAL_Driver/Inc/stm32f7xx_hal_i2c.h:559:19: note: expected 'uint8_t * {aka unsigned char *}' but argument is of type 'volatile uint8_t * {aka volatile unsigned char *}'
 HAL_StatusTypeDef HAL_I2C_Slave_Sequential_Transmit_IT(I2C_HandleTypeDef *hi2c, uint8_t *pData, uint16_t Size, uint32_t XferOptions);

                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
hardware/STM32/stm32/cores/arduino/stm32/twi.c:558:49: warning: passing argument 2 of 'HAL_I2C_Slave_Sequential_Receive_IT' discards 'volatile' qualifier from pointer target type [-Wdiscarded-qualifiers]
       HAL_I2C_Slave_Sequential_Receive_IT(hi2c, &(obj->i2cTxRxBuffer[obj->slaveRxNbData]),
                                                 ^
In file included from hardware/STM32/stm32/variants/DISCO_F746NG/stm32f7xx_hal_conf.h:330:0,
                 from hardware/STM32/stm32/system/Drivers/STM32F7xx_HAL_Driver/Inc/stm32f7xx_hal.h:46,
                 from hardware/STM32/stm32/system/Drivers/CMSIS/Device/ST/STM32F7xx/Include/stm32f7xx.h:219,
                 from hardware/STM32/stm32/cores/arduino/stm32/stm32_def.h:18,
                 from hardware/STM32/stm32/cores/arduino/stm32/twi.c:51:
hardware/STM32/stm32/system/Drivers/STM32F7xx_HAL_Driver/Inc/stm32f7xx_hal_i2c.h:560:19: note: expected 'uint8_t * {aka unsigned char *}' but argument is of type 'volatile uint8_t * {aka volatile unsigned char *}'
 HAL_StatusTypeDef HAL_I2C_Slave_Sequential_Receive_IT(I2C_HandleTypeDef *hi2c, uint8_t *pData, uint16_t Size, uint32_t XferOptions);

                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
hardware/STM32/stm32/cores/arduino/stm32/twi.c: In function 'HAL_I2C_ListenCpltCallback':
hardware/STM32/stm32/cores/arduino/stm32/twi.c:579:31: warning: passing argument 1 of 'obj->i2c_onSlaveReceive' discards 'volatile' qualifier from pointer target type [-Wdiscarded-qualifiers]
       obj->i2c_onSlaveReceive(obj->i2cTxRxBuffer, obj->slaveRxNbData);
                               ^~~
hardware/STM32/stm32/cores/arduino/stm32/twi.c:579:31: note: expected 'uint8_t * {aka unsigned char *}' but argument is of type 'volatile uint8_t * {aka volatile unsigned char *}'
hardware/STM32/stm32/cores/arduino/stm32/twi.c: In function 'HAL_I2C_SlaveRxCpltCallback':
hardware/STM32/stm32/cores/arduino/stm32/twi.c:604:49: warning: passing argument 2 of 'HAL_I2C_Slave_Sequential_Receive_IT' discards 'volatile' qualifier from pointer target type [-Wdiscarded-qualifiers]
       HAL_I2C_Slave_Sequential_Receive_IT(hi2c, &(obj->i2cTxRxBuffer[obj->slaveRxNbData]),
                                                 ^
In file included from hardware/STM32/stm32/variants/DISCO_F746NG/stm32f7xx_hal_conf.h:330:0,
                 from hardware/STM32/stm32/system/Drivers/STM32F7xx_HAL_Driver/Inc/stm32f7xx_hal.h:46,
                 from hardware/STM32/stm32/system/Drivers/CMSIS/Device/ST/STM32F7xx/Include/stm32f7xx.h:219,
                 from hardware/STM32/stm32/cores/arduino/stm32/stm32_def.h:18,
                 from hardware/STM32/stm32/cores/arduino/stm32/twi.c:51:
hardware/STM32/stm32/system/Drivers/STM32F7xx_HAL_Driver/Inc/stm32f7xx_hal_i2c.h:560:19: note: expected 'uint8_t * {aka unsigned char *}' but argument is of type 'volatile uint8_t * {aka volatile unsigned char *}'
 HAL_StatusTypeDef HAL_I2C_Slave_Sequential_Receive_IT(I2C_HandleTypeDef *hi2c, uint8_t *pData, uint16_t Size, uint32_t XferOptions);

                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[LMESTM]

@fpistm I think it would be ok to cast it. I added volatile to make sure that the buffer being filled in in the function i2c_slave_write_IT will not be optimized out by the compiler.
// Check the communication status
for(i = 0; i < size; i++) {
obj->i2cTxRxBuffer[i] = *(data+i);
}

[fpistm]

PR #355 merged