[media] v4l: ov772x: simplify pointer dereference

Use a more direct way to obtain a pointer to struct ov772x_priv, where the
subdevice is available.

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