staging: emxx_udc: add SPDX identifier.

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

Fix up the staging emxx_udc driver to have a proper SPDX identifier,
based on the license text in the file itself.  The SPDX identifier is a
legally binding shorthand, which can be used instead of the full boiler
plate text.

This work is based on a script and data from Thomas Gleixner, Philippe
Ombredanne, and Kate Stewart.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/emxx_udc/emxx_udc.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  *  drivers/usb/gadget/emxx_udc.c
  *     EMXX FCD (Function Controller Driver) for USB.

drivers/staging/emxx_udc/emxx_udc.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  *  EMXX FCD (Function Controller Driver) for USB.
  *