net: dpaa2: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header files related to DPAA2 Ethernet driver supporting
Freescale SoCs with DPAA2. 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>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/dpaa2/dpaa2-ptp.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright 2018 NXP
  */

drivers/net/ethernet/freescale/dpaa2/dprtc-cmd.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright 2013-2016 Freescale Semiconductor Inc.
  * Copyright 2016-2018 NXP

drivers/net/ethernet/freescale/dpaa2/dprtc.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright 2013-2016 Freescale Semiconductor Inc.
  * Copyright 2016-2018 NXP