flag immediately so it's only set once per longcal interval.
Without this, the current AR5416 code will continuously spam NF
calibrations during a periodic calibration if the longcal flag
is set. The longcal flag wouldn't be cleared until the calibration
method indicates that calibrations are "complete".
This drops the rate of NF calibration updates down from "once every
shortcal" (ie, every 100ms) during a periodic calibration, to only
once per "longcal" interval. Spamming NF calibrations every 100ms
caused some potentially horrific issues in noisy environments as
NF calibrations can take longer than 100ms and this spamming can
cause invalid NF calibration results to be read back - leading to
missed beacons, and thus leading to a stuck beacon situation.
Stuck beacons cause interface resets, which restart calibrations.
This means that the longcal calibration runs every 100ms (shortcal)
until all initial calibrations are completed. This spamming can then
cause the above issues which leads to stuck beacons, leading to
interface resets, etc, etc. Quite annoying.
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