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

This patch corrects the SPDX License Identifier style in
header file related to early USB devices.
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: NJoe Perches <joe@perches.com>
Signed-off-by: NNishad Kamdar <nishadkamdar@gmail.com>
Link: https://lore.kernel.org/r/20200328104426.GA6401@nishadSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/early/xhci-dbc.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * xhci-dbc.h - xHCI debug capability early driver
  *