SImplefy STBC handler

This patchset removes unused code and use ATH_RC_TX_STBC_FLAG instead of
ATH_RC_RX_STBC_FLAG to enable STBC streaming.
Known issues:
- If AP has changed supported flags. For example "NO RxSTBC" to "1RX STBC", firmware may have not time to notify host driver. It will just reconnect without updating capflags.