serial: ns16550: Use old baud rate divisor for flushing if not given

If baud_divisor is not set (i.e. == -1), we should use the baud divisor
already in use for flushing the xmit register. If we don't flush the
xmit register, then SPL will hang.

Signed-off-by: Patrik Dahlström <risca@dalakolonin.se>