virtio-mmio: fix missing put_device() when vm_cmdline_parent registration failed

The reference must be released when device_register(&vm_cmdline_parent)
failed. Add the corresponding 'put_device()' in the error handling path.
Signed-off-by: chengkaitao <pilgrimtao@gmail.com>
Message-Id: <20220602005542.16489-1-chengkaitao@didiglobal.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/virtio/virtio_mmio.c

@@ -701,6 +701,7 @@ static int vm_cmdline_set(const char *device,
 	if (!vm_cmdline_parent_registered) {
 		err = device_register(&vm_cmdline_parent);
 		if (err) {
+			put_device(&vm_cmdline_parent);
 			pr_err("Failed to register parent device!\n");
 			return err;
 		}