[ALSA] usb-audio - allow type 0 extension units

USB generic driver
Extension units can have type 0, so do not ignore them when constructing
mixer controls.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
diff --git a/sound/usb/usbmixer.c b/sound/usb/usbmixer.c
index bfaec4f..5683ae0 100644
--- a/sound/usb/usbmixer.c
+++ b/sound/usb/usbmixer.c
@@ -1232,9 +1232,6 @@
 	}
 
 	type = combine_word(&dsc[4]);
-	if (! type)
-		return 0; /* undefined? */
-
 	for (info = list; info && info->type; info++)
 		if (info->type == type)
 			break;