media: async: Warn on attept to create an ancillary link to a non-subdev

Warn if a lens or a flash async sub-device is bound to a non-sub-device
notifier. This isn't meaningful and unlikely to happen in practice but
print a warning as we nevertheless won't consider this an error.

Signed-off-by: Sakari Ailus <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

drivers/media/v4l2-core/v4l2-async.c

···
 	    sd->entity.function != MEDIA_ENT_F_FLASH)
 		return 0;
 
-	if (!n->sd)
+	if (!n->sd) {
+		dev_warn(notifier_dev(n),
+			 "not a sub-device notifier, not creating an ancillary link for %s!\n",
+			 dev_name(sd->dev));
 		return 0;
+	}
 
 	link = media_create_ancillary_link(&n->sd->entity, &sd->entity);