sound: usb-audio: do not make URBs longer than sync packet interval

Using more packets in one URB do avoid interrupts does not make sense
when we have a sync pipe whose packets generate interrupts more often.
Therefore, limit the URB size to the synchronization packet interval.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usbaudio.c

@@ -1083,6 +1083,8 @@ static int init_substream_urbs(struct snd_usb_substream *subs, unsigned int peri
 	} else
 		urb_packs = 1;
 	urb_packs *= packs_per_ms;
+	if (subs->syncpipe)
+		urb_packs = min(urb_packs, 1 << subs->syncinterval);
 
 	/* decide how many packets to be used */
 	if (is_playback) {