ALSA: usb - For class 2 devices, use channel map from altsettings

The channel config from the streaming descriptor is probably a
better indicator of the channel map than the input terminal.
Use the input terminal's channel map as fallback only.
Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/stream.c

@@ -587,6 +587,7 @@ int snd_usb_parse_audio_interface(struct snd_usb_audio *chip, int iface_no)
 
 			num_channels = as->bNrChannels;
 			format = le32_to_cpu(as->bmFormats);
+			chconfig = le32_to_cpu(as->bmChannelConfig);
 
 			/* lookup the terminal associated to this interface
 			 * to extract the clock */
@@ -594,7 +595,8 @@ int snd_usb_parse_audio_interface(struct snd_usb_audio *chip, int iface_no)
 									    as->bTerminalLink);
 			if (input_term) {
 				clock = input_term->bCSourceID;
-				chconfig = le32_to_cpu(input_term->bmChannelConfig);
+				if (!chconfig && (num_channels == input_term->bNrChannels))
+					chconfig = le32_to_cpu(input_term->bmChannelConfig);
 				break;
 			}