iio:dac:ad5686: Change license description

Add GPLv2+ SPDX identifier and remove license notice to keep the whole
purpose of using an SPDx id.
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5686.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * AD5686R, AD5685R, AD5684R Digital to analog converters  driver
  *
  * Copyright 2011 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
  */
 
 #include <linux/interrupt.h>