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

This patch corrects the SPDX License Identifier style in
header files related to STMicroelectronics based Multi-Gigabit
Ethernet 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: 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/stmicro/stmmac/dwmac5.h

-// SPDX-License-Identifier: (GPL-2.0 OR MIT)
+/* SPDX-License-Identifier: (GPL-2.0 OR MIT) */
 // Copyright (c) 2017 Synopsys, Inc. and/or its affiliates.
 // stmmac Support for 5.xx Ethernet QoS cores
 

drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h

-// SPDX-License-Identifier: (GPL-2.0 OR MIT)
+/* SPDX-License-Identifier: (GPL-2.0 OR MIT) */
 /*
  * Copyright (c) 2018 Synopsys, Inc. and/or its affiliates.
  * stmmac XGMAC definitions.

drivers/net/ethernet/stmicro/stmmac/hwif.h

-// SPDX-License-Identifier: (GPL-2.0 OR MIT)
+/* SPDX-License-Identifier: (GPL-2.0 OR MIT) */
 // Copyright (c) 2018 Synopsys, Inc. and/or its affiliates.
 // stmmac HW Interface Callbacks