[netdevice] Fix failure path in register_netdev()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdevice.c

 	clear_settings ( netdev_settings ( netdev ) );
 	unregister_settings ( netdev_settings ( netdev ) );
  err_register_settings:
+	list_del ( &netdev->list );
+	netdev_put ( netdev );
  err_duplicate:
 	return rc;
 }