mxser: use port variable in mxser_set_serial_info

mxser_set_serial_info already defined a local 'port' of type struct
tty_port. So use it in the code everywhere.

This eliminates some text from the code there.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-65-jslaby@suse.cz


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