[CFT] STBC is enabled for AR9280 and later

[Adrian Chadd]

Hiya,

I've just enabled STBC TX and RX on the AR9280 and later.

* STBC TX requires two chains to work right. So enabling a single
chain only or the single chain NICs (AR9285) won't work;
* STBC RX works with one chain but you really need two chains to get
the benefit;
* STBC is only supported for single-stream rates (MCS 0 -> 7.)

So, for AR9280 and AR9287 users, STBC should work for you now out of
the box. AR9285 users - you can receive STBC but it won't give you any
benefits.

The AR5416, AR9130 and AR9160 NICs don't support STBC.

When the AR9300 HAL is released, STBC will work for all of those NICs.

What does it mean? It should give increased sensitivity/performance at
the lower MCS rates which should translate to a generally better wifi
experience.

THanks,


Adrian