[ath_hal] fixes for finer grain timestamping, some 11n macros

• 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.