[media] dvb-usb: 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.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>