media: ov2740: Use sub-device active state

Use sub-device active state. Rely on control handler lock to serialise
access to the active state. Also clean up locking on s_stream handler.

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>