USB: core: add missing license information to some files

Some of the USB core files were missing explicit license information.
As all files in the kernel tree are implicitly licensed under the
GPLv2-only, be explicit in case someone get confused looking at
individual files by using the SPDX nomenclature.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/buffer.c

@@ -3,6 +3,9 @@
  *
  * This implementation plugs in through generic "usb_bus" level methods,
  * and should work with all USB controllers, regardless of bus type.
+ *
+ * Released under the GPLv2 only.
+ * SPDX-License-Identifier: GPL-2.0
  */
 
 #include <linux/module.h>