USB: audio gadget: fix wTotalLength calculation

The wTotalLength should contain the sum of the interface and unit
descriptor sizes per the Audio Device Class specification 1.0.

Reported-by: Sergey Lapin <slapin@ossfans.org>
Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>