iio: magnetometer: st_magn: fix drdy line configuration for LIS3MDL

Data-ready line in LIS3MDL is routed to drdy pin and it is not possible
to select a different INT pin. st_sensors_set_dataready_irq() assumes
that if drdy int address is not exported in register map, irq trigger
is not supported by the sensor and hw_irq_trigger is always false.
Based on this configuration st_sensors_irq_thread does not consume
generated interrupt causing an unhandled irq.
Fix this taking into account status register address in
st_sensors_set_dataready_irq()

Fixes: 90efe055 (iio: st_sensors: harden interrupt handling)
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>