sfc: add missing licence info to mcdi_filters.c

Both the licence notice and the SPDX tag were missing from this file.
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/mcdi_filters.c

+// SPDX-License-Identifier: GPL-2.0-only
+/****************************************************************************
+ * Driver for Solarflare network controllers and boards
+ * Copyright 2005-2018 Solarflare Communications Inc.
+ * Copyright 2019-2020 Xilinx Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published
+ * by the Free Software Foundation, incorporated herein by reference.
+ */
+
 #include "mcdi_filters.h"
 #include "mcdi.h"
 #include "nic.h"