RFT: deferred/driver management of sequence number allocation

[Adrian Chadd]

hi,

https://reviews.freebsd.org/D9195

This is an initial patch to allow drivers to handle sequence number
allocation. This is important for higher throughput because it
eliminates the need for the TX serialisation lock in net80211 for
those drivers.

It's only done for ath(4) right now.

I'd appreciate testing by people. Hopefully it's a big no-op for
everyone, including ath(4) users!

Thanks,

-adrian