[media] v4l2-device: Don't unregister ACPI/Device Tree based devices

[media] v4l2-device: Don't unregister ACPI/Device Tree based devices

When a V4L2 sub-device backed by a DT or ACPI based device was removed,
the device was unregistered as well which certainly was not intentional,
as the client device would not be re-created by simply reinstating the
V4L2 sub-device (indeed the device would have to be there first!).

Skip unregistering the device in case it has non-NULL of_node or fwnode.

Signed-off-by: Tommi Franttila <tommi.franttila@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>