iio: imu: st_lsm6dsx: inline per-sensor data

As it has been already done for other st sensors in 'commit 9049531c
("iio: accel: st_accel: inline per-sensor data")', get rid of some
defines and just open code the values into the appropriate struct
elements since the semantic meaning is inherent in the name of the
C99-addressable fields and there is no reason to duplicate the code

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