[media] dvb_usb_v2: attach tuners later

It is more than nice to have all those frontend pointers when
attaching tuner to the frontend in question. Make that possible
attaching tuners after dvb_register_frontend()

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