Reorder HardwareSerial initialisation list to fix compiler warnings

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 the following compiler warning (repeated for each of the `HardwareSerial*.cpp` source files the header is used in) introduced as part of the improvements in #1711:

```
In file included from HardwareSerial.cpp:31:
HardwareSerial.h: In constructor 'HardwareSerial::HardwareSerial(volatile uint8_t*, volatile uint8_t*, volatile uint8_t*, volatile uint8_t*, volatile uint8_t*, volatile uint8_t*)':
HardwareSerial.h:81: warning: 'HardwareSerial::_tx_buffer_tail' will be initialized after
HardwareSerial.h:78: warning:   'volatile uint8_t HardwareSerial::_rx_buffer_head'
HardwareSerial_private.h:62: warning:   when initialized here
```