mxser: introduce and use start/stop_tx helpers

mxser: introduce and use start/stop_tx helpers

Starting and stopping TX is performed on many places in the driver. It
is repeated outb and info->IER modification.  Create few helpers
(__mxser_start_tx, mxser_start_tx, __mxser_stop_tx) and use them in the
code instead of open coding.

This should be a base for an uart driver for this device in the future.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-60-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>