media: v4l: subdev: Return routes set using S_ROUTING

Return the routes set using S_ROUTING back to the user. Also reflect this
in documentation.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>