[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.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>