[ath_rate_sample] Have the final attempted rate in 11n modes to be the lowest one.

Right now ath_rate_sample has a fixed rate schedule, rather than the minstrel_ht
style "best, good, most reliable" triplet. So, if higher rates are tried then
it'll not fail back to a lower MCS rate in that transmission schedule.

This means that in low SNR situations it'll not easily drop to MCS0 unless enough
transmissions occur to allow rate control to eventually decide to drop; and if
it's TCP traffic it'll get slowed down because of packet loss.

It's worse for 2-stream and 3-stream rates; it doesn't ever fall back to lower
stream rates, and these higher stream rates required higher SNR to work.

So instead let's (for now?) have each of the 11n transmit rates use MCS0 as
the last attempt. ath_rate_sample will quickly see that rate succeeds more
and will move to it much quicker.

Testing:

• AR9344 (Wasp) - 2G STA mode