initial scan offload patch

[Adrian Chadd]

hiya,

Here's my first cut at teaching the software scan about full offload firmware.

https://reviews.freebsd.org/D8262

This:

* adds a flag to say "full offload!"
* disables sending probe requests - firmware can do that
* disables doing powersave management - again, firmware can do that.

This is intended for the intel (wpi, iwn, iwm) firmware that can do
it. And definitely for the upcoming ath10k driver I'm porting.

Thanks!


-adrian