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).
Merge pull request #1835 from ribbons/hardwareserial-warning

Reorder HardwareSerial initialisation list to fix compiler warnings