[net80211] add STBC capability flags to iv_flags_ht.

This is in preparation for exposing configuring STBC flags up to ifconfig
so STBC TX/RX can be configured at runtime.

• Set the FHT_STBC flags for TX/RX if the HT capabilitiex exist
• Clear the RX STBC HT capability flag when creating a HTCAP IE, so we only announce it if it's configured in the FHT flags.

Tested:

• AR9331 (carambola2), AP/STA modes