iwn(4) hangs after r257133

Turns out that not enabling MRR causes my Intel Ultimate N WiFi Link
5300 to hang after only a few moments of use.

For now, I've just reverted only those aspects of r257133, enabling
MRR and keeping the rate index lookup, which seems to do something on
my hardware at least (I assume it's not the right thing based on
Adrian's analysis, but it works never-the-less).

Has anyone else hit this with Intel WiFi hardware?

Also, what needs to be done to have MRR working properly?

-Brandon