[media] dvb_usb_v2: pass (struct dvb_usb_device *) as a parameter for fw download

Change parameter (struct usb_device *) => (struct dvb_usb_device *)
for .identify_state() and .download_firmware() callbacks.

struct usb_device * did not provide handle for the DVB USB driver state.
Change DVB USB framework to alloc space	for the	priv earlier and pass
that pointer to the device driver using (struct dvb_usb_device *)
as a callback parameter.

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