[media] ov772x: Make to_ov772x convert from v4l2_subdev to ov772x_priv
Conversion from i2c_client to ov772x_priv is only needed in a single
location, while conversion from v4l2_subdev to ov772x_priv is needed in
several locations. Perform the former manually, and use to_ov772x for
the later.