[media] dvb_usb_v2: implement .get_firmware_name()

Use callback to return firmware name instead of static firmware name.
There is some chips that needs to select firmware name according to
chip revision.

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