iio: magnetometer: st_magn: remove ihl property for LSM303AGR

Remove IRQ active low support for LSM303AGR since the sensor does not
support that capability for data-ready line

Fixes: a9fd053b

 (iio: st_sensors: support active-low interrupts)
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>