usbback: fix after c/s 1232:8806dfb939d4

That c/s ("backends: Check for insane amounts of requests on the ring")
copied from blkback/blktap/scsiback a switch statement that is valid
there, but not here - the return value from usbbk_start_submit_urb()
could be any positive value, not just one.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/usbback/usbback.c
+++ b/drivers/xen/usbback/usbback.c
@@ -1013,7 +1013,7 @@ static int usbbk_start_submit_urb(usbif_
 
 	RING_FINAL_CHECK_FOR_REQUESTS(&usbif->urb_ring, more_to_do);
 
-	return more_to_do;
+	return !!more_to_do;
 }
 
 void usbbk_hotplug_notify(usbif_t *usbif, int portnum, int speed)