firmware: arm_scmi: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style
in header file related to Firmware Drivers for ARM SCMI
Message Protocol.
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/2019/2/7/46Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/firmware/arm_scmi/common.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * System Control and Management Interface (SCMI) Message Protocol
  * driver common header file containing some definitions, structures