tty: vcc: Drop impossible to hit WARN_ON

vcc_get() returns the port that has provided port->index. As the port that
is about to be removed isn't removed yet this trivially will find this
port. So simplify the call to not assign an identical value to the port
pointer and drop the warning that is never hit.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>