mxser: remove unused mxser_port::stop_rx

It's been a long time since mxser_port::stop_rx was abandoned. Remove it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-35-jslaby@suse.cz


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