ALSA: sound: Make static

Sparse asked whether these could be static.
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usx2y/usb_stream.c

@@ -276,7 +276,8 @@ static void subs_set_complete(struct urb **urbs, void (*complete)(struct urb *))
 	}
 }
 
-int usb_stream_prepare_playback(struct usb_stream_kernel *sk, struct urb *inurb)
+static int usb_stream_prepare_playback(struct usb_stream_kernel *sk,
+		struct urb *inurb)
 {
 	struct usb_stream *s = sk->s;
 	struct urb *io;