Fixed main bugs that made transmitting with UART work.

- Added code to initialise the _linbtr variable that was missing.
- On the LIN UART there is no "Tx Buffer Empty" flag, only a flag
  that triggers an interrupt when the last transfer is complete.
  Modified Serial::write to kick-start transmission process if there
  is currently no active transfer taking place.
- Other general tidying up to the modified code sections.
Corrected UART parity setting defines.

Defines for UART parity options (in LINCR LCONF[1:0]) were wrong.

Now corrected.