iio: imu: st_lsm6dsx: fix decimation factor estimation

Fix decimation factor and sip estimation for LSM6DSM series
(max value for decimation factor is 32).
If gyro and accel sensors are enabled at 12.5Hz and 416Hz
respectively, decimation factor lookup will fail, producing
unaligned data.
Remove unused decimator filed in st_lsm6dsx_sensor structure.

Fixes: f8710f03 ("iio: imu: st_lsm6dsx: express odr in mHZ")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>