iio: imu: st_lsm6dsx: move fs_table in st_lsm6dsx_sensor_settings

Move fs_table in st_lsm6dsx_sensor_settings in order to support
sensors with different gain maps. This is a preliminary patch to add
support for LSM9DS1 sensor to st_lsm6dsx driver

Tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>