driver/sensor: lis3mdl: Fix DRDY interrupt

The LIS3MDL sensor provides two different pins for handling
interrupts: the DRDY, that triggers new data sample availability,
and INT, that goes off when data sample exceeds a given threshold.

The driver handled data ready triggers only, which does not
require sensor configuration at all. Moreover a dummy read is
required when the data ready is configured to re-trigger a new
interrupt.

Signed-off-by: Armando Visconti <armando.visconti@st.com>