ALSA: usb-audio: Fix parsing descriptor of UAC2 processing unit

commit a6618f4a upstream.

Currently, the offsets in the UAC2 processing unit descriptor are
calculated incorrectly. It causes an issue when connecting the device which
provides such a feature:

~~~~
[84126.724420] usb 1-1.3.1: invalid Processing Unit descriptor (id 18)
~~~~

After this patch is applied, the UAC2 processing unit inits w/o this error.

Fixes: 23caaf19 ("ALSA: usb-mixer: Add support for Audio Class v2.0")
Signed-off-by: Kirill Marinushkin <k.marinushkin@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>