[media] dvb_usb_v2: implement .get_adapter_count()

Callback to resolve adapter count of current device.
Old static .num_adapters field can be still used but
the new .get_adapter_count() has priority if both
offered by the driver.

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