[media] dvb_frontend: add "detach" callback

Prepare for making "release" asynchronous (via kref).  Some operations
may need to be run synchronously in dvb_frontend_detach(), and that's
why we need a "detach" callback.

Signed-off-by: Max Kellermann <max.kellermann@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>