mxser: call stop_rx from mxser_shutdown_port()

mxser_stop_rx() should be called from mxser_shutdown_port() for several
reasons:
1) info->slock is held while manipulating IER (as on other places),
2) hangup now stops rx too,
3) mxser_close() will use tty_port_close() and there is no place except
   tty_port_operations::shutdown() where this can be done,
4) this is the same sequence as serial_core does. So we can map this
   code 1:1 when switching the driver to it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-9-jslaby@suse.cz


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