Reorder HardwareSerial init to fix compiler warn

Switch the tx and rx buffer head/tail entries in the HardwareSerial
initialisation list so that they match the order the fields are defined
in. This fixes a compiler warning (repeated for each of the
HardwareSerial source files the header is used in).

hardware/arduino/avr/cores/arduino/HardwareSerial_private.h

@@ -63,8 +63,8 @@ HardwareSerial::HardwareSerial(
     _ubrrh(ubrrh), _ubrrl(ubrrl),
     _ucsra(ucsra), _ucsrb(ucsrb), _ucsrc(ucsrc),
     _udr(udr),
-    _tx_buffer_head(0), _tx_buffer_tail(0),
-    _rx_buffer_head(0), _rx_buffer_tail(0)
+    _rx_buffer_head(0), _rx_buffer_tail(0),
+    _tx_buffer_head(0), _tx_buffer_tail(0)
 {
 }