mxser: cleanup mxser_change_speed

* use UART macros instead of magic constants
* let the default case reuse already existing code (CS5 case)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-38-jslaby@suse.cz


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