USB: add SPDX identifiers to all remaining Makefiles

It's good to have SPDX identifiers in all files to make it easier to
audit the kernel tree for correct licenses.

This updates the remaining drivers/usb/*Makefile* that were missing SPDX
identifiers. They all get the following identifier:
SPDX-License-Identifier: GPL-2.0

The SPDX identifier is a legally binding shorthand, which can be used
instead of the full boiler plate text.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Peter Korsgaard <jacmet@sunsite.dk>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: Johan Hovold <johan@kernel.org>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Linux-2.6.12-rc2

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!