[bwn] accurately(ish) account transmit/recieve failures for rate control.

[bwn] accurately(ish) account transmit/recieve failures for rate control.

I noticed that it'd associate fine, but it'd quickly stop exchanging traffic.
Receive was okay, but transmit just failed.

Then I went "wlandebug +rate".  I discovered it started at 36M OFDM, and then
quickly rose to 54M, which then showed 0% transmit success.

Then, I dug into how the completion path works.  We are reading 'ack=0'
in the TX status side, so .. then I discovered we were only processing the
TX completion status /if/ ack=1.  So, we'd only ever count successes;
we'd never count failures, and thus the rate control code thought
everything was a-ok.

We also have to set retrycnt to something non-zero so it indeed does
bring the rate down upon failure.

So:

* Delete the rate control completion code from the tx completion
  routine, it's just duplicate and never worked.  Putting it behind
  'if (status->ack) was pointless.

* Move it to the PIO and DMA completion routines which actually
  do free the node reference and mbuf.  We know at that point
  what the status is, so do it there.

* Fake a retrycnt of 1 for now, so we at least count failures.

Also:

* Start adding comments about weird stuff I find with rate selection.
  In this instance, we shouldn't be selecting a fallback rate that
  doesn't match the currently configured mode (11a, 11b, 11g, etc.)

This isn't perfect - AMRR does try 54mbit and takes a few packets
before it figures out it's a bad idea - but it's better than nothing.

This makes the bwn(4) driver actually useful for the first time since
I've tried using it - and that dates back to 2011.  I've resisted
successfully until now.

Tested:

* Broadcom BCM4312 802.11b/g Wireless, STA mode
  WLAN (chipid 0x4312 rev 15) PHY (analog 6 type 5 rev 1) RADIO (manuf 0x17f ver 0x2062 rev 2)

TODO:

* See if the fallback rate actually /is/ working
* Question my own sanity over touching this driver in the first place.