staging: iio: ad7780: add SPDX identifier

Add SPDX identifier (GPL-2.0) to the AD7780 driver.
Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/adc/ad7780.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD7170/AD7171 and AD7780/AD7781 SPI ADC driver
  *
  * Copyright 2011 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
  */
 
 #include <linux/interrupt.h>