ALSA: usb - Don't trust the channel config if the channel count changed

In case the channel count of the input terminal is not the same as
the channel count of the streaming descriptor, the channel config of
the input terminal can not be trusted. Instead fall back to a default
(guessed) channel map.

This was found on a Logitech USB Headset.
Signed-off-by: NDavid Henningsson <david.henningsson@canonical.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/usb/stream.c

@@ -662,7 +662,6 @@ int snd_usb_parse_audio_interface(struct snd_usb_audio *chip, int iface_no)
 					* (fp->maxpacksize & 0x7ff);
 		fp->attributes = parse_uac_endpoint_attributes(chip, alts, protocol, iface_no);
 		fp->clock = clock;
-		fp->chmap = convert_chmap(num_channels, chconfig, protocol);
 
 		/* some quirks for attributes here */
 
@@ -698,12 +697,16 @@ int snd_usb_parse_audio_interface(struct snd_usb_audio *chip, int iface_no)
 		/* ok, let's parse further... */
 		if (snd_usb_parse_audio_format(chip, fp, format, fmt, stream) < 0) {
 			kfree(fp->rate_table);
-			kfree(fp->chmap);
 			kfree(fp);
 			fp = NULL;
 			continue;
 		}
 
+		/* Create chmap */
+		if (fp->channels != num_channels)
+			chconfig = 0;
+		fp->chmap = convert_chmap(fp->channels, chconfig, protocol);
+
 		snd_printdd(KERN_INFO "%d:%u:%d: add audio endpoint %#x\n", dev->devnum, iface_no, altno, fp->endpoint);
 		err = snd_usb_add_audio_stream(chip, stream, fp);
 		if (err < 0) {