[media] dvb_usb_v2: probe/disconnect error handling

In my understanding we never call .disconnect() when .probe()
returns error. Change .probe() to return error all cases it cannot
handle given interface and simplify .disconnect() handling.

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