drivers/sensor/st: iis2iclx: (fix) convert range into mg

Fix the sensor range setting at runtime which is currently returning error,
converting the SI (ms2) value passed thru SENSOR_ATTR_FULL_SCALE in mg (and
not in g) as this is the unit used in the driver internals.

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