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 <[e-mail]>
Signed-off-by: Nishad Kamdar <[e-mail]>
Link: https://lore.kernel.org/r/14bb[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

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