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>