[ath] [ath_rate] Add some extra data into the rate control lookup.

Right now (well, since I did this in 2011/2012) the rate control code
makes some super bad choices for 11n aggregates/rates, and it tracks
statistics even more questionably.

It's been long enough and I'm now trying to use it again daily, so let's
start by:

• telling the rate control code if it's an aggregate or not;
• being clearer about the TID - yes it can be extracted from the ath_buf but this way it can be overridden by the caller without changing the TID itself.
  
  (This is for doing experiments with voice/video QoS at some point..)

• Return an optional field to limit how long the aggregate is in microseconds. Right now the rate control code supplies a rate table and the ath aggr form code will look at the rate table and limit the aggregate size to 4ms at the slowest rate. Yeah, this is pretty terrible.

• Add some more TODO comments around handling txpower, rate and handling filtered frames status so if I continue to have spoons for this I can go poke at it.