ALSA: usb-audio - Fix types taken in min()

Fix the compile warning due to different integer types used in min():
  sound/usb/usbaudio.c: In function 'init_substream_urbs':
  sound/usb/usbaudio.c:1087: warning: comparison of distinct pointer types lacks a cast
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usbaudio.c

@@ -1084,7 +1084,7 @@ static int init_substream_urbs(struct snd_usb_substream *subs, unsigned int peri
 		urb_packs = 1;
 	urb_packs *= packs_per_ms;
 	if (subs->syncpipe)
-		urb_packs = min(urb_packs, 1 << subs->syncinterval);
+		urb_packs = min(urb_packs, 1U << subs->syncinterval);
 
 	/* decide how many packets to be used */
 	if (is_playback) {