virtio_mmio: Set dev.release() to avoid warning

Fix a warning thrown from virtio_mmio_remove():
Device 'virtio0' does not have a release() function

The fix is according to virtio_pci_probe() of
drivers/virtio/virtio_pci_common.c

Signed-off-by: Yuan Liu <liuyuan@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mmio.c

@@ -489,6 +489,7 @@ static const struct virtio_config_ops virtio_mmio_config_ops = {
 };
 
 
+static void virtio_mmio_release_dev_empty(struct device *_d) {}
 
 /* Platform device */
 
@@ -511,6 +512,7 @@ static int virtio_mmio_probe(struct platform_device *pdev)
 		return  -ENOMEM;
 
 	vm_dev->vdev.dev.parent = &pdev->dev;
+	vm_dev->vdev.dev.release = virtio_mmio_release_dev_empty;
 	vm_dev->vdev.config = &virtio_mmio_config_ops;
 	vm_dev->pdev = pdev;
 	INIT_LIST_HEAD(&vm_dev->virtqueues);