The 11n duration calculation function in net80211 and the HAL round /up/
the duration calculation for short-gi, so we can't use that.
The 11n duration calculation doesn't know about the extra symbol time
needed for STBC, nor the LDPC encoding duration, so we can't use
that.
This (along with other, local hacks) allow the locationing services to
get down to around 200nS (yes, nanoseconds) of variance when speaking
to a "good" AP.
Tested:
* AR9380, STA mode, local locationing frame hacks
ffa25bec32