[csl] avoid retransmissions of a frame with CSL IE

[testter2]

As described in this issue 6976 , For retransmissions of a frame with CSL IE, the phase is set just on first try (and the frame secured).
For the next tries, the frame remains the same, having invalid phase.

so why can't we just prevent retransmissions of a frame with CSL IE so it would create new frame again with correct phase ?

[jwhui]

The Thread Specification was updated to address this issue - see SPEC-977 for details.

We still need to implement the latest specified behavior. I have included it as a TODO on the Thread Spec Implementation project page.

[testter2]

Thanks @jwhui