virtio_ring: adapt to notify() returning bool

Correct if statement to check for bool returned by notify()
(introduced in 5b1bf7cb67).

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_ring.c

@@ -441,7 +441,7 @@ bool virtqueue_notify(struct virtqueue *_vq)
 		return false;
 
 	/* Prod other side to tell it about changes. */
-	if (vq->notify(_vq) < 0) {
+	if (!vq->notify(_vq)) {
 		vq->broken = true;
 		return false;
 	}