Convert net80211 over to using if_transmit for the dispatch from the
upper layer(s).

This eliminates the if_snd queue from net80211. Yay!

This unfortunately has a few side effects:

* It breaks ALTQ to net80211 for now - sorry everyone, but fixing
  parallelism and eliminating the if_snd queue is more important
  than supporting this broken traffic scheduling model. :-)

* There's no VAP and IC flush methods just yet - I think I'll add
  some NULL methods for now just as placeholders.

* It reduces throughput a little because now net80211 will drop packets
  rather than buffer them if the driver doesn't do its own buffering.
  This will be addressed in the future as I implement per-node software
  queues.

Tested:

* ath(4) and iwn(4) in STA operation