media: ov2740: Enable runtime PM before registering the async subdev

Enable runtime PM before registering the async subdev as the driver UAPI
may become accessible immediately after the registration. Runtime PM needs
to be enabled by that time.

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>