[media] dvb-usb-v2: add frontend_detach callback

Add frontend_detach callback in order to allow custom detach. It is
needed when demod driver is implemented I2C client or some other
kernel bus, but not proprietary dvb_attach / dvb_detach.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>