mxser: use tty_port_close() in mxser_close()

Finally, the mxser_close() code in is mostly identical to
tty_port_close(), so replace the code by a single call to the function.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-11-jslaby@suse.cz


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