[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: NClemens Ladisch <clemens@ladisch.de>

sound/usb/usbmixer.c

@@ -1232,9 +1232,6 @@ static int build_audio_procunit(mixer_build_t *state, int unitid, unsigned char
 	}
 
 	type = combine_word(&dsc[4]);
-	if (! type)
-		return 0; /* undefined? */
-
 	for (info = list; info && info->type; info++)
 		if (info->type == type)
 			break;