staging: fwserial: 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 fwserial 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/fwserial/dma_fifo.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * DMA-able FIFO implementation
  *

drivers/staging/fwserial/dma_fifo.h

+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * DMA-able FIFO interface
  *

drivers/staging/fwserial/fwserial.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * FireWire Serial driver
  *