[media] v4l2-dev: vdev->v4l2_dev is always set, so simplify code

These days vdev->v4l2_dev must always be set. This means that some
old code that still tests for a NULL vdev->v4l2_dev can be removed
or simplified.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>