USB: gadget: audio: provide correct device id

The audio gadget driver should use a "Linux" device id, instead
of relying on NetChip's vendor id.  So provide one out of our reserved
namespace.

Cc: Bryan Wu <cooloney@kernel.org>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>