Enable slow diversity combining for the AR9285.

Enable slow diversity combining for the AR9285.

Now that I understand what's going on - and the RX antenna array maps
to what the receive LNA configuration actually is - I feel comfortable
in enabling this.

If people do have issues with this, there's enough debugging now available
that we have a chance to diagnose it without writing it up as 'weird
crap.'

Tested:

* AR9285 STA w/ diversity combining enabled in EEPROM

TODO:

* (More) testing in hostap mode