[media] dvb_usb_v2: move 3 callbacks from the frontend to adapter

Move .frontend_attach(), .tuner_attach() and .streaming_ctrl()
from the frontend to adapter.

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