platform/chrome: cros_ec_proto: switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>

drivers/platform/chrome/cros_ec_proto.c

-/*
- * ChromeOS EC communication protocol helper functions
- *
- * Copyright (C) 2015 Google, Inc
- *
- * This software is licensed under the terms of the GNU General Public
- * License version 2, as published by the Free Software Foundation, and
- * may be copied, distributed, and modified under those terms.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- */
+// SPDX-License-Identifier: GPL-2.0
+// ChromeOS EC communication protocol helper functions
+//
+// Copyright (C) 2015 Google, Inc
 
 #include <linux/mfd/cros_ec.h>
 #include <linux/delay.h>