mxser: make mxser_port::ldisc_stop_rx a bool

It's used only as a yes-no variable, so make it a bool and switch the
set values appropriately.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210922075938.31390-3-jslaby@suse.cz


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