[media] omap3isp: Avoid a BUG_ON() in media_entity_create_link()

If an uninitialised v4l2_subdev struct was passed to
media_entity_create_link(), one of the BUG_ON()'s in the function will be
hit since media_entity.num_pads will be zero. Avoid this by checking whether
the num_pads field is non-zero for the interface.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>