[media] dvbdev: add a dvb_detach() macro

The dvb_attach() was unbalanced, as there was no dvb_dettach. Ok,
on current cases, the dettach is done by dvbdev, but that are some
future corner cases where we may need to do this before registering
the frontend.

So, add a dvb_detach() and use it at dvb_frontend.c.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>