tty: riscom8, fix shutdown declaration

tty_port_ops.shutdown takes only one parameter: tty port. Remove
the second one and use port->tty where needed instead.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>