Don't use driver_version from struct media_device, just return
LINUX_VERSION_CODE as the other media subsystems do.

The driver_version field in struct media_device will be removed
in the following patches.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>