[ath] obey the STBC flag setting in iv_flags_ht

Add support for the FHT_STBC_TX flag in iv_flags_ht, so it'll now obey
the per-vap ifconfig stbctx flag.

This means that we can do STBC TX on one vap and not another VAP.
(As well as STBC RX on said vap; that changes the HTCAP announcement.)