USB: mtu3: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header files related to MediaTek USB3 Dual Role controller.
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>
Reviewed-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20200404103728.GA6011@nishadSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/mtu3/mtu3.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * mtu3.h - MediaTek USB3 DRD header
  *

drivers/usb/mtu3/mtu3_debug.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * mtu3_debug.h - debug header
  *

drivers/usb/mtu3/mtu3_dr.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * mtu3_dr.h - dual role switch and host glue layer header
  *

drivers/usb/mtu3/mtu3_hw_regs.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * mtu3_hw_regs.h - MediaTek USB3 DRD register and field definitions
  *

drivers/usb/mtu3/mtu3_qmu.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * mtu3_qmu.h - Queue Management Unit driver header
  *

drivers/usb/mtu3/mtu3_trace.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /**
  * mtu3_trace.h - trace support
  *