iio: make iio_device_get_drvdata take a const struct iio_dev *.

As this just calls dev_get_drvdata underneath which is happy with
a const struct device * we should change and avoid potentially
casting away a const in order to then put it back again.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/iio.h

@@ -669,7 +669,7 @@ static inline void iio_device_set_drvdata(struct iio_dev *indio_dev, void *data)
  *
  * Returns the data previously set with iio_device_set_drvdata()
  */
-static inline void *iio_device_get_drvdata(struct iio_dev *indio_dev)
+static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev)
 {
 	return dev_get_drvdata(&indio_dev->dev);
 }