V4L/DVB: v4l: Use v4l2_get_subdevdata instead of accessing v4l2_subdev::priv

Replace direct access to the v4l2_subdev priv field with the inline
v4l2_get_subdevdata method.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>