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

This patch corrects the SPDX License Identifier style
in header files related to Cavium Ethernet 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: Joe Perches <joe@perches.com>
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cavium/common/cavium_ptp.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /* cavium_ptp.h - PTP 1588 clock on Cavium hardware
  * Copyright (c) 2003-2015, 2017 Cavium, Inc.
  */