mxser: rename mxser_close_port() to mxser_stop_rx()

As it is the only thing it does now. This is one of the future
serial_core hooks.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-3-jslaby@suse.cz


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