media: v4l: subdev: Store the sub-device in the sub-device state

Store the sub-device in the sub-device state. This will be needed in e.g.
validating pad number when retrieving information for non-stream-aware
users. There are expected to be more needs for this in the future.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>