[iwm] fix up the rate control setup code to initialise rates in the order
we want to use it.

The rate table was being initialised in low->high, but the link quality
table was being initialised high->low. So, when we did a lookup, we
would get the indexes wrong.

This started by a patch from dragonflybsd which reversed how the ni->in_ridx[]
array is being used; I'd rather it all be consistent. So, this is consistent.

Inspired by: what I did to iwn(4) a while ago
Inspired by: DragonflyBSD; <imre@vdsz.com>