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

commit b500c086 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>