siox: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header file related to Eckelmann SIOX 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>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Thorsten Scherer <thorsten.scherer@eckelmann.de>
Link: https://lore.kernel.org/r/20200101131418.GA3110@nishadSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/siox/siox.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2015-2017 Pengutronix, Uwe Kleine-König <kernel@pengutronix.de>
  */