iio:common:st_sensors: add st_sensors_get_settings_index() helper function

Extract from st_sensors_check_device_support() function the code that
is used to get the specific settings for a device. This will be used
as generic extractor by each ST driver.

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>