ALSA: usb - Add missing KERN_* prefix to printk

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/usbmixer.c b/sound/usb/usbmixer.c
index 330f2fb..6615cd3 100644
--- a/sound/usb/usbmixer.c
+++ b/sound/usb/usbmixer.c
@@ -222,7 +222,10 @@
 	for (p = state->map; p->id; p++) {
 		if (p->id == unitid && ! p->name &&
 		    (! control || ! p->control || control == p->control)) {
-			// printk("ignored control %d:%d\n", unitid, control);
+			/*
+			printk(KERN_DEBUG "ignored control %d:%d\n",
+			       unitid, control);
+			*/
 			return 1;
 		}
 	}