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

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: 23caaf19b11e ("ALSA: usb-mixer: Add support for Audio Class v2.0")
Signed-off-by: Kirill Marinushkin <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

include/uapi/linux/usb/audio.h

···
 {
 	return (protocol == UAC_VERSION_1) ?
 		desc->baSourceID[desc->bNrInPins + 4] :
-		desc->baSourceID[desc->bNrInPins + 6];
+		2; /* in UAC2, this value is constant */
 }
 
 static inline __u8 *uac_processing_unit_bmControls(struct uac_processing_unit_descriptor *desc,
···
 {
 	return (protocol == UAC_VERSION_1) ?
 		&desc->baSourceID[desc->bNrInPins + 5] :
-		&desc->baSourceID[desc->bNrInPins + 7];
+		&desc->baSourceID[desc->bNrInPins + 6];
 }
 
 static inline __u8 uac_processing_unit_iProcessing(struct uac_processing_unit_descriptor *desc,