V4L/DVB: bt819: cleanup v4l2_subdev_notify() parameters

The 3rd parameter v4l2_subdev_notify() is passed to the notify() callback
which is a pointer, see <media/v4l2-subdev.h> and <media/v4l2-device.h>.

This will remove the following sparse warning (see "make C=1"):
 * Using plain integer as NULL pointer

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>