[media] dvb-usb-v2: create one media_dev per adapter

Instead of assuming just one adapter, change the code to store
one media controller per adapter.

This works fine for dvb-usb, as, on all drivers here, it is not
possible to write a media graph that would mix resources between
the two different adapters.

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

drivers/media/usb/dvb-usb-v2/dvb_usb.h

@@ -392,10 +392,6 @@ struct dvb_usb_device {
 	struct delayed_work rc_query_work;
 
 	void *priv;
-
-#if defined(CONFIG_MEDIA_CONTROLLER_DVB)
-	struct media_device *media_dev;
-#endif
 };
 
 extern int dvb_usbv2_probe(struct usb_interface *,