iio: mma8452: use iio event type IIO_EV_TYPE_MAG

IIO_EV_TYPE_THRESH in rising direction describes an event where the
threshold is crossed in rising direction, positive or negative values
being possible. This is not the case here.

Since the threshold is no signed value and only the magnitude is compared,
IIO_EV_TYPE_MAG is what describes the behaviour of these devices, see the
sysfs-bus-iio ABI Documentation.

Signed-off-by: Martin Kepplinger <martin.kepplinger@theobroma-systems.com>
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>