m68knommu: fix coldfire tcdrain

Fix tcdrain on coldfire uarts.
Currently with coldfire uarts tcdrain returns without waiting for txempty,
because (tx)fifosize is 0.  Fix that and call uart_update_timeout when
setting the baud rate, otherwise tcdrain will wait for an half our :)
Also constify mcf_uart_ops.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>