virtio: don't crash when device is buggy

Because of a sanity check in virtio_dev_remove, a buggy device can crash
kernel.  And in case of rproc it's userspace so it's not a good idea.
We are unloading a driver so how bad can it be?
Be less aggressive in handling this error: if it's a driver bug,
warning once should be enough.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio.c

@@ -159,7 +159,7 @@ static int virtio_dev_remove(struct device *_d)
 	drv->remove(dev);
 
 	/* Driver should have reset device. */
-	BUG_ON(dev->config->get_status(dev));
+	WARN_ON_ONCE(dev->config->get_status(dev));
 
 	/* Acknowledge the device's existence again. */
 	add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);