[media] v4l: of: Support empty port nodes

Empty port nodes are allowed but currently unsupported as the
v4l2_of_get_next_endpoint() function assumes that all port nodes have at
least an endpoint. Fix this.

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