[media] drxk: Don't assume a default firmware name

Move the ngene/ddbridge firmware into their drivers.

There are two reasons for that:
	1) The firmware used there didn't work for a few devices
I tested here (Terratec H5, H6 and H7);
	2) At least Terratec H7 doesn't seem to require a firmware
for it to work.

After this change, if firmware is not specified, the driver will
use a rom-based firmware (this seems to be the case for Terratec
H7, although I need to better check the USB dumps to be sure about
that).

In any case, the firmware seems to be optional, as the DRX-K driver
don't return the firmware load error.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>