feat: sets TX buffer

This will allow to set TX buffer to a minimum with no error message.
It also makes it works as in the Arduino API specification that is to return the buffer available space. In ESP32 case it will be the minmum the HW TX FIFO size of 128 bytes.

Author: SuGlider

Diff for: cores/esp32/HardwareSerial.cpp

@@ -562,13 +562,12 @@ size_t HardwareSerial::setTxBufferSize(size_t new_size) {
         return 0;
     }
 
-    if (new_size < SOC_UART_FIFO_LEN) {
+    if (new_size <= SOC_UART_FIFO_LEN) {
         log_w("TX Buffer set to minimum value: %d.", SOC_UART_FIFO_LEN);  // ESP32, S2, S3 and C3 means higher than 128
         _txBufferSize = 0; // it will use just UART FIFO with SOC_UART_FIFO_LEN bytes (128 for most SoC)
         return SOC_UART_FIFO_LEN;
     }
-    // if new_size is SOC_UART_FIFO_LEN, _txBufferSize will be zero - just use the UART FIFO space
-    _txBufferSize = new_size - SOC_UART_FIFO_LEN; // for total correct report from "availableForWrite()"  that matches a call to this function
+    // if new_size is higher than SOC_UART_FIFO_LEN, TX Ringbuffer will be active and it will be used to report back "availableToWrite()"
+    _txBufferSize = new_size;
     return new_size;
 }
-