Use correct type for index calculation in HardwareSerial

Author: cmaglie

Diff for: hardware/arduino/avr/cores/arduino/HardwareSerial_private.h

@@ -99,7 +99,7 @@ void HardwareSerial::_rx_complete_irq(void)
     // No Parity error, read byte and store it in the buffer if there is
     // room
     unsigned char c = *_udr;
-    uint8_t i = (unsigned int)(_rx_buffer_head + 1) % SERIAL_RX_BUFFER_SIZE;
+    rx_buffer_index_t i = (unsigned int)(_rx_buffer_head + 1) % SERIAL_RX_BUFFER_SIZE;
 
     // if we should be storing the received character into the location
     // just before the tail (meaning that the head would advance to the