to become idle; it actually doesn't matter as the chip takes care of
this for us. It would also spin endlessly in certain circumstances
during shutdown.
While I'm here, (finally) add code to allow sending BREAK characters.
when deciding if the baud-rate should be changed. (Spotted by Bruce Martin)
Also fix the mask used when setting the Rx FIFO threshold to zero. I don't
yet understand how the Rx side worked with the code as it was. ;-}
was deasserted and we wanted to change parameters to -crtscts
This effectively stalled the transmitter since with TS_BUSY set
we only change parameters during the Tx interrupt handler.
