environments.
In setups where NF calibration can take a while, don't load the CCA
and kick off a new NF calibration if the previous one hasn't yet
completed. This shouldn't happen unless the environment is noisy but
those exist (hi phk!).
Here, if the previous NF hasn't completed when ar5416LoadNf() is run
(which reads the NF), it skips updating the history buffer, loading
the NF CCA array and kicking off the next NF cal. It's hoped it'll
occur in the next long calibration interval.
Obtained from: Atheros, ath9k, my local HAL
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