Fixed FIFO buffer declarations & UARTA0 examples.

- UARTA0 code had an example of how to insert it into an ISR. The ISR code was
  for TI's compiler. It has been switched to match the declaration of an ISR for
  GCC.
- FIFO buffers did not have the count marked as volatile. This caused the UART
  code to get caught in an infinite loop if the buffer ever filled up, as it
  would check the count without expecting it to ever change. This has been
  fixed, and it now expects an ISR could change it at any time.