iio: st_sensors: do not always write enable_axis register

New devices (e.g. LIS2DW12) enable all axis by default and do not export
that capability in register map. Check if the enable_axis register
address has been declared in st_sensor_settings map in order to verify if
the driver needs to enable all sensor axis

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>