mxser: move FIFO clearing to mxser_disable_and_clear_FIFO()

This code is used on two places, make a helper for disabling and
clearing FIFOs. And demangle the too complicated 'if' by using a local
variable.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210922075938.31390-5-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>