media: uvcvideo: Handle cameras with invalid descriptors

If the source entity does not contain any pads, do not create a link.

Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>