iio: envelope-detector: switch to SPDX license identifier

Drop the boilerplate license text.
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/envelope-detector.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Driver for an envelope detector using a DAC and a comparator
  *
  * Copyright (C) 2016 Axentia Technologies AB
  *
  * Author: Peter Rosin <peda@axentia.se>
- *
- * 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.
  */
 
 /*