After much toying around with this AMRR initial rate stuff,
I've decided that for 11n rates it's best to start (very) low and work
our way up.

So, from now on, the initial rate for AMRR 11n is MCS4.
It doesn't try MCS12 or MCS20 - at low signal strengths those don't
work very well at all.

AMRR will step the rate control up over time if things work out better.

Tested:

* Intel 5100
* Intel 5300 (using local diffs to test out 3x3 stream support)