media: v4l2-subdev: Document that routing support depends on streams

Routing support, through the subdev .set_routing() operation, requires
the subdev to support streams. This is however not clearly documented
anywhere. Fix it by expanding the operation's documentation to indicate
that subdevs must set the V4L2_SUBDEV_FL_STREAMS flag.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

include/media/v4l2-subdev.h

@@ -822,8 +822,9 @@ struct v4l2_subdev_state {
  *		     operation shall fail if the pad index it has been called on
  *		     is not valid or in case of unrecoverable failures.
  *
- * @set_routing: enable or disable data connection routes described in the
- *		 subdevice routing table.
+ * @set_routing: Enable or disable data connection routes described in the
+ *		 subdevice routing table. Subdevs that implement this operation
+ *		 must set the V4L2_SUBDEV_FL_STREAMS flag.
  *
  * @enable_streams: Enable the streams defined in streams_mask on the given
  *	source pad. Subdevs that implement this operation must use the active