[media] v4l: subdev: Clean up properly in subdev devnode registration error path

Set the subdev devnode pointer right after registration to ensure that
later errors won't skip the subdev when unregistering all devnodes.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>