iio: imu: inv_icm42600: avoid frequent timestamp jitter

We are currently synchronizing every time the data timestamp with
the IT timestamp, leading to system jitter jamming timestamps.
To fix that and keep it simple, let's just synchronize when the
delta is bigger than the acceptable jitter, and keep
synchronization at the jitter value.

The result is much stable timestamps reflecting better the real
physical value. Example @50Hz delta timestamp,
* before: 20.123ms, 19.721ms, 20.023ms, 20.353ms, 19.821ms, ...
* after: 20.173ms, 20.173ms, 20.173ms, 20.40ms, 20.173ms, ...

Refactorize code and delete the unnecessary handling of multiple
FIFO data.

Signed-off-by: Jean-Baptiste Maneyrol <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Cameron <[email protected]>

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