iio: imu: st_lsm6dsx: set sensor->odr value just in st_lsm6dsx_write_raw()

Update odr value in st_lsm6dsx_sensor data structure just in
st_lsm6dsx_write_raw() in order to avoid to set the same value
multiple times

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>