[media] dvb_usb_v2: move (struct usb_data_stream) to one level up

Move stream from the frontend to adapter. There could be only
one stream per adapter. One adapter can has multiple frontends
but only one can stream at the time.

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