virtio_net: Fix error code in probe()

Set a negative error code intead of returning success if the MTU has
been changed to something invalid.

Fixes: fe36cbe0 ("virtio_net: clear MTU when out of range")
Reported-by: Robert Buhren <robert.buhren@sect.tu-berlin.de>
Reported-by: Felicitas Hetzelt <file@sect.tu-berlin.de>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/X8pGVJSeeCdII1Ys@mwandaSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/net/virtio_net.c

@@ -3072,6 +3072,7 @@ static int virtnet_probe(struct virtio_device *vdev)
 			dev_err(&vdev->dev,
 				"device MTU appears to have changed it is now %d < %d",
 				mtu, dev->min_mtu);
+			err = -EINVAL;
 			goto free;
 		}