Broken OT 1.2 support in openthread/ot-nrf528xx

[George-Stefan]

It looks like for retransmissions of a frame with CSL IE and security enabled are corrupted.
Since the CSL IE phase is updated after the frame was secured.

See nrf_802154_tx_started() from https://github.com/openthread/ot-nrf528xx/blob/main/src/src/radio.c#L1216

[edmont]

Hi @George-Stefan, thanks for spotting this. This PR should fix it: #149

Please notice that even after the fix the CSL IEs will be outdated for the retransmitted frames. This is a known issue and Thread Group is currently discussing a proper approach to address it.

[edmont]

In any case OpenThread has a workaround already for the out of sync issue: openthread/openthread#6342