virtio: don't set VIRTIO_CONFIG_S_DRIVER_OK twice.

I noticed this with the console device.  It's not *wrong*, just a bit
weird.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio.c

@@ -236,7 +236,10 @@ static int virtio_dev_probe(struct device *_d)
 	if (err)
 		goto err;
 
-	add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
+	/* If probe didn't do it, mark device DRIVER_OK ourselves. */
+	if (!(dev->config->get_status(dev) & VIRTIO_CONFIG_S_DRIVER_OK))
+		virtio_device_ready(dev);
+
 	if (drv->scan)
 		drv->scan(dev);