USB: add missing SPDX lines to Kconfig and Makefiles

There are a few remaining drivers/usb/ files that do not have SPDX
identifiers in them, all of these are either Kconfig or Makefiles.  Add
the correct GPL-2.0 identifier to them to make scanning tools happy.

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