iio: 104-quad-8: Update license boilerplate

This patch simplifies the boilerplate license text by making use of a
SPDX license identifier line.

Signed-off-by: William Breathitt Gray <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/iio/counter/104-quad-8.c

···
+// SPDX-License-Identifier: GPL-2.0
 /*
  * IIO driver for the ACCES 104-QUAD-8
  * Copyright (C) 2016 William Breathitt Gray
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License, version 2, as
- * published by the Free Software Foundation.
- *
- * This program is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- * General Public License for more details.
  *
  * This driver supports the ACCES 104-QUAD-8 and ACCES 104-QUAD-4.
  */