[media] dvb_usb_v2: implement .read_config()

That callback is called	only once when device is connected.
Call is done after the possible firmware is downloaded to the device,
just after the .power_ctrl() and before adapters are created.

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