[media] v4l: of: Drop acquired reference to node when getting next endpoint

The of_get_child_by_name() function takes a reference to the node it
returns. Make sure to drop it when looking for the ports node in
v4l2_of_get_next_endpoint().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>