net: dsa: b53: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style
in header file for Broadcom BCM53xx managed switch 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>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

drivers/net/dsa/b53/b53_serdes.h

-/* SPDX-License-Identifier: GPL-2.0 or BSD-3-Clause
- *
+/* SPDX-License-Identifier: GPL-2.0 or BSD-3-Clause */
+/*
  * Northstar Plus switch SerDes/SGMII PHY definitions
  *
  * Copyright (C) 2018 Florian Fainelli <f.fainelli@gmail.com>