[media] dvb_usb_v2: use identify_state() to resolve firmware name

Merge get_firmware_name() to identify_state().

It is wise to resolve firmware name in that routine as it does
decision wether or not to load firmware at all. It is one very
rarely needed callback less.

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