[media] media/core: Replace ctrl_class with which

Replace the obsolete field ctrl_class with "which".

Make sure it not used in future modules by commenting out the field with
ifndef __KERNEL_ .

The field cannot be simply removed because that would be change on the
kenel API to the userspace (and we don't like that).

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>