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.

---
 drivers/usb/early/xhci-dbc.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 673686eeddd7..9fb6a9f2548a 100644
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * xhci-dbc.h - xHCI debug capability early driver
  *

2.17.1