pinctrl: stm32: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header file related to STMicroelectronics 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/14bb695da50f7af8499e7dfc32c2ab753d92a3e9.1574871463.git.nishadkamdar@gmail.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/stm32/pinctrl-stm32.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) Maxime Coquelin 2015
  * Copyright (C) STMicroelectronics 2017