staging: iio: adc: ad7606: Use SPDX identifier

This patch replaces the license text at the top of ad7606 driver files
and instead adds SPDX GPL-2.0 license identifier.
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/adc/ad7606.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD7606 SPI ADC driver
  *
  * Copyright 2011 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
  */
 
 #include <linux/interrupt.h>

drivers/staging/iio/adc/ad7606.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * AD7606 ADC driver
  *
  * Copyright 2011 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
  */
 
 #ifndef IIO_ADC_AD7606_H_

drivers/staging/iio/adc/ad7606_par.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD7606 Parallel Interface ADC driver
  *
  * Copyright 2011 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
  */
 
 #include <linux/module.h>

drivers/staging/iio/adc/ad7606_spi.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD7606 SPI ADC driver
  *
  * Copyright 2011 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
  */
 
 #include <linux/module.h>