net: dsa: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style
in header file for Distributed Switch Architecture drivers.
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: NJoe Perches <joe@perches.com>
Signed-off-by: NNishad Kamdar <nishadkamdar@gmail.com>
Reviewed-by: NVivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>

drivers/net/dsa/lantiq_pce.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * PCE microcode extracted from UGW 7.1.1 switch api
  *