staging: iio: ad5933: add SPDX identifier

Add SPDX identifier of GPL-2.0 for the ad5933 driver.
Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/impedance-analyzer/ad5933.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD5933 AD5934 Impedance Converter, Network Analyzer
  *
  * Copyright 2011 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
  */
 
 #include <linux/clk.h>