* change the HT_RC_2_MCS to do MCS0..23
* Use it when looking up the ht20/ht40 array for bits-per-symbol
* add a clk_to_psec (picoseconds) routine, so we can get sub-microsecond
accuracy for the math
* .. and make that + clk_to_usec public, so higher layer code that is
returning clocks (eg the ANI diag routines, some upcoming locationing
experiments) can be converted to microseconds.
Whilst here, add a comment in ar5416 so i or someone else can revisit the
latency values.
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