drivers: iio: dac: Fix wrong license for ADI drivers

Analog Devices drivers are typically GPL v2 only. This patch fixes the
inconsistencies between the module license and SPDX.
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5686-spi.c

-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD5672R, AD5674R, AD5676, AD5676R, AD5679R,
  * AD5681R, AD5682R, AD5683, AD5683R, AD5684,

drivers/iio/dac/ad5686.c

-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD5686R, AD5685R, AD5684R Digital to analog converters  driver
  *

drivers/iio/dac/ad5686.h

-/* SPDX-License-Identifier: GPL-2.0+ */
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * This file is part of AD5686 DAC driver
  *

drivers/iio/dac/ad5696-i2c.c

-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD5671R, AD5675R, AD5691R, AD5692R, AD5693, AD5693R,
  * AD5694, AD5694R, AD5695R, AD5696, AD5696R

drivers/iio/dac/ad5758.c

-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD5758 Digital to analog converters driver
  *