[net80211] document the (not completely complete) set of places where
we're assuming hz=1000 and not gracefully handling when it isn't.

The math involved will return 0 for hz < 1000, which it is on some
platforms and on DragonflyBSD.

This doesn't fix it, it:

• converts one manual use over to use the macro, and
• comments where it needs some thought/fixing.

I'll think about this a bit more before fixing it.

Submitted by: imre@vdsz.com