[ALSA] usb-audio - use proper interval between synchronization packets

USB generic driver
Add sanity checks when reading the bRefresh value, and actually use it
for the synchronization packets instead of polling at 1000 Hz.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

sound/usb/usbaudio.c

@@ -1051,10 +1051,7 @@ static int init_substream_urbs(snd_usb_substream_t *subs, unsigned int period_by
 			u->urb->pipe = subs->syncpipe;
 			u->urb->transfer_flags = URB_ISO_ASAP;
 			u->urb->number_of_packets = u->packets;
-			if (snd_usb_get_speed(subs->dev) == USB_SPEED_HIGH)
-				u->urb->interval = 8;
-			else
-				u->urb->interval = 1;
+			u->urb->interval = 1 << subs->syncinterval;
 			u->urb->context = u;
 			u->urb->complete = snd_usb_complete_callback(snd_complete_sync_urb);
 		}
@@ -1272,7 +1269,12 @@ static int set_format(snd_usb_substream_t *subs, struct audioformat *fmt)
 			subs->syncpipe = usb_rcvisocpipe(dev, ep);
 		else
 			subs->syncpipe = usb_sndisocpipe(dev, ep);
-		subs->syncinterval = get_endpoint(alts, 1)->bRefresh;
+		if (get_endpoint(alts, 1)->bLength >= USB_DT_ENDPOINT_AUDIO_SIZE &&
+		    get_endpoint(alts, 1)->bRefresh >= 1 &&
+		    get_endpoint(alts, 1)->bRefresh <= 9)
+			subs->syncinterval = get_endpoint(alts, 1)->bRefresh;
+		else
+			subs->syncinterval = 1;
 	}
 
 	/* always fill max packet size */