[media] v4l subdev: add dispatching for VIDIOC_DBG_G_REGISTER and VIDIOC_DBG_S_REGISTER

Ioctls on the subdevs node currently don't dispatch the register access debug
driver callbacks. Add the dispatching with the same security checks are for
non subdev video nodes (i.e. only capable(CAP_SYS_ADMIN may call the register
access ioctls).

Signed-off-by: Martin Hostettler <martin@neutronstar.dyndns.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>