[media] dvb_usb_v2: restore .firmware - pointer to name

Most commonly only one firmware is used by the driver
and it is not needed to selected run time. So restore
old functionality but allow .get_firmware_name() callback
to override it.

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