Don't enable the HT flags for legacy rates.

I stumbled across this whilst trying to debug another weird hang reported
on the freebsd-wireless list.

Whilst here, add in the STBC check to ath_rateseries_setup().

Whilst here, fix the short preamble flag to be set only for legacy rates.

Whilst here, comment that we should be using the full set of decisions
made by ath_rateseries_setup() rather than recalculating them!
Oops - fix an incorrect test.
Add missing flags.
Minor wordsmithing.

X-MFC-Needs:	r245617