mxser: remove tty->driver_data NULL check

Noone sets tty->driver_data to NULL in the driver, so there is no point
to check that in mxser_close(). Remove the check.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-8-jslaby@suse.cz


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