USB: dwc2: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header files related to DesignWare USB2 DRD Core Support.
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: Felipe Balbi <balbi@kernel.org>

drivers/usb/dwc2/core.h

-// SPDX-License-Identifier: (GPL-2.0+ OR BSD-3-Clause)
+/* SPDX-License-Identifier: (GPL-2.0+ OR BSD-3-Clause) */
 /*
  * core.h - DesignWare HS OTG Controller common declarations
  *

drivers/usb/dwc2/debug.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * debug.h - Designware USB2 DRD controller debug header
  *

drivers/usb/dwc2/hcd.h

-// SPDX-License-Identifier: (GPL-2.0+ OR BSD-3-Clause)
+/* SPDX-License-Identifier: (GPL-2.0+ OR BSD-3-Clause) */
 /*
  * hcd.h - DesignWare HS OTG Controller host-mode declarations
  *

drivers/usb/dwc2/hw.h

-// SPDX-License-Identifier: (GPL-2.0+ OR BSD-3-Clause)
+/* SPDX-License-Identifier: (GPL-2.0+ OR BSD-3-Clause) */
 /*
  * hw.h - DesignWare HS OTG Controller hardware definitions
  *