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

This patch corrects the SPDX License Identifier style in
header files related to PHY Layer for 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). This patch also gives an explicit
block comment to the SPDX License Identifier.

Changes made by using a script provided by Joe Perches here:
https://lkml.org/lkml/2019/2/7/46.

Suggested-by: Joe Perches <[e-mail]>
Signed-off-by: Nishad Kamdar <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/phy/aquantia.h

···
-/* SPDX-License-Identifier: GPL-2.0
- * HWMON driver for Aquantia PHY
+/* SPDX-License-Identifier: GPL-2.0 */
+/* HWMON driver for Aquantia PHY
  *
  * Author: Nikita Yushchenko <[e-mail]>
  * Author: Andrew Lunn <[e-mail]>

drivers/net/phy/bcm-phy-lib.h

···
-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2015 Broadcom Corporation
  */

drivers/net/phy/mdio-cavium.h

···
-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2009-2016 Cavium, Inc.
  */

drivers/net/phy/mdio-i2c.h

···
-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * MDIO I2C bridge
  *

drivers/net/phy/mdio-xgene.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /* Applied Micro X-Gene SoC MDIO Driver
  *
  * Copyright (c) 2016, Applied Micro Circuits Corporation