[media] dvb-usb-v2: add pointer to 'struct usb_interface' for driver usage

Top level pointer on USB probe is struct usb_interface *. Add that
pointer to struct dvb_usb_device that drivers could use it, for
dev_* logging and more.

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