drivers: sensor: lsm6dsl: add .attr_set callback

In order to be able to change at runtime the Output Data
Rate (ODR) as well as the Full-Scale (FS) for both the
Accelerometr and Gyroscope the driver needs to provide
the .attr_set callback.

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