ALSA: usb-audio: ADC3: Fix channel mapping conversion for ADC3.

The channel mapping is defined by bChRelationship, not bChPurpose.

Fixes: 9a2fe9b801f5 ("ALSA: usb: initial USB Audio Device Class 3.0 support")
Reviewed-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Michael Drake <michael.drake@codethink.co.uk>
Signed-off-by: Jorge Sanjuan <jorge.sanjuan@codethink.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/stream.c

@@ -349,7 +349,7 @@ snd_pcm_chmap_elem *convert_chmap_v3(struct uac3_cluster_header_descriptor
 			 * TODO: this conversion is not complete, update it
 			 * after adding UAC3 values to asound.h
 			 */
-			switch (is->bChPurpose) {
+			switch (is->bChRelationship) {
 			case UAC3_CH_MONO:
 				map = SNDRV_CHMAP_MONO;
 				break;