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>
