iio: common: st_sensors: check odr address value in st_sensors_set_odr()

Do not try to configure sample frequency if the sensor do not export
odr register address in register map. That change will be used to
properly support LIS3DHH accel sensor.

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