Revert "Fixed writable for LPC1768"

This reverts commit 19ae097c62efb66eeaa7c8ba8778a9b26d092fe3.
Fixed LPC1768 writable

Now if no flow control is used it will check if holding register is
empty, instead of complete UART is empty: The TxIrq uses the same
requirement.
Currently BufferedSerial (and everyone else trying something similar)
goes haywire because it doesn't just check if writable, but also if done
writing.

If flow control is enabled nothing is changed.