iio: magnetometer: st_magn: fix status register address for LSM303AGR

Fixes: 97865fe4 (iio: st_sensors: verify interrupt event to status)
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>