drivers/sensor: lsm6dsl: use sensor APIs to convert to units

Make use of sensor_ug_to_ms2() and sensor_10udegrees_to_rad() APIs
to convert from raw values into proper units (i.e. m/s^2 for accel
and rad/s for gyro). It also improves measurement precision.

Fixes #63980

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