in preparation for the scan based retune logic.
The linux iwlwifi driver does a rescan (onto a non-active channel)
to force an RF retune when the PLCP error rates exceed a certain threshold.
* Add code to track HT PLCP rate errors;
* Separate out the PLCP error count fetch and update so the delta
can be used when checking for PLCP error rates;
* Implement the PLCP error logic from iwlwifi;
* For now, just print out whenever the error rate exceeds the
threshold.
The actual scan based retune will take a bit more effort; the scan
command code right now assumes that a scan state is passed in.
This does need to change to be more flexible (both for this and
in preparation for scanning multiple channels at once.)
Tested:
* 5100 (STA mode)
* 2200 (STA mode)
* 2230 (STA mode)
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