pinctrl: mediatek: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header file related mediatek mt2712 pinctrl driver.
For C header files Documentation/process/license-rules.rst
mandates C-like comments (opposed to C source files where
C++ style should be used).

Changes made by using a script provided by Joe Perches here:
https://lkml.org/lkml/2019/2/7/46.
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Link: https://lore.kernel.org/r/2994fb2f3375790e832396cdbb0a279dc8c8839f.1574871463.git.nishadkamdar@gmail.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/mediatek/pinctrl-mtk-mt2712.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2018 MediaTek Inc.
  * Author: Zhiyong Tao <zhiyong.tao@mediatek.com>