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