tty/serial: make port of serial8250_register_8250_port const

After the previous patch, we can make port passed to
serial8250_find_match_or_unused const. And then we can make const also
port of serial8250_register_8250_port.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210519072153.3859-2-jslaby@suse.cz


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