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

This patch corrects the SPDX License Identifier style in
header files related to USB Serial device configuration.
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 <[e-mail]>
Signed-off-by: Nishad Kamdar <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>

drivers/usb/serial/belkin_sa.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Definitions for Belkin USB Serial Adapter Driver
  *

drivers/usb/serial/io_16654.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	16654.H		Definitions for 16C654 UART used on EdgePorts

drivers/usb/serial/io_edgeport.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	io_edgeport.h	Edgeport Linux Interface definitions

drivers/usb/serial/io_ionsp.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	IONSP.H		Definitions for I/O Networks Serial Protocol

drivers/usb/serial/io_ti.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*****************************************************************************
  *
  *	Copyright (C) 1997-2002 Inside Out Networks, Inc.

drivers/usb/serial/io_usbvend.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	USBVEND.H		Vendor-specific USB definitions

drivers/usb/serial/iuu_phoenix.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Infinity Unlimited USB Phoenix driver
  *

drivers/usb/serial/mct_u232.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Definitions for MCT (Magic Control Technology) USB-RS232 Converter Driver
  *

drivers/usb/serial/oti6858.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Ours Technology Inc. OTi-6858 USB to serial adapter driver.
  */

drivers/usb/serial/pl2303.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Prolific PL2303 USB to serial adaptor driver header file
  */

drivers/usb/serial/visor.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * USB HandSpring Visor driver
  *

drivers/usb/serial/whiteheat.h

···
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * USB ConnectTech WhiteHEAT driver
  *