USB: dwc3: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header files related to DesignWare USB3 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/dwc3/core.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * core.h - DesignWare USB3 DRD Core Header
  *

drivers/usb/dwc3/debug.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /**
  * debug.h - DesignWare USB3 DRD Controller Debug Header
  *

drivers/usb/dwc3/gadget.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * gadget.h - DesignWare USB3 DRD Gadget Header
  *

drivers/usb/dwc3/io.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /**
  * io.h - DesignWare USB3 DRD IO Header
  *

drivers/usb/dwc3/trace.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /**
  * trace.h - DesignWare USB3 DRD Controller Trace Support
  *