feat: adjust availableForWrite

This change will make sure that if no TX Ringbuffer is used, it will return the UART FIFO available space. Otherwise, it will return the Ringbuffer available space, as defined in the Arduino especification.

Author: SuGlider

Diff for: cores/esp32/esp32-hal-uart.c

@@ -642,7 +642,7 @@ uint32_t uartAvailableForWrite(uart_t* uart)
     uint32_t available =  uart_ll_get_txfifo_len(UART_LL_GET_HW(uart->num));  
     size_t txRingBufferAvailable = 0;
     if (ESP_OK == uart_get_tx_buffer_free_size(uart->num, &txRingBufferAvailable)) {
-        available += txRingBufferAvailable; 
+        available = txRingBufferAvailable == 0 ? available : txRingBufferAvailable; 
     }
     UART_MUTEX_UNLOCK();
     return available;