iio: magnetometer: ak8974: Fix negative raw values in sysfs

At the moment, reading from in_magn_*_raw in sysfs tends to return
large values around 65000, even though the output of ak8974 is actually
limited to ±32768. This happens because the value is never converted
to the signed 16-bit integer variant.

Add an explicit cast to s16 to fix this.

Fixes: 7c94a8b2 ("iio: magn: add a driver for AK8974")
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Waleij <linus.walleij@linaro.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/magnetometer/ak8974.c

@@ -564,7 +564,7 @@ static int ak8974_read_raw(struct iio_dev *indio_dev,
 		 * We read all axes and discard all but one, for optimized
 		 * reading, use the triggered buffer.
 		 */
-		*val = le16_to_cpu(hw_values[chan->address]);
+		*val = (s16)le16_to_cpu(hw_values[chan->address]);
 
 		ret = IIO_VAL_INT;
 	}