Convert ath(4) to just use ieee80211_suspend_all() and ieee80211_resume_all().

The existing code tries to use the beacon miss timer to signal that the AP
has gone away. Unfortunately this doesn't seem to be behaving itself.
I'll try to investigate why this is for the sake of completeness.

The result is the STA will stay "associated" to the AP it was associated
with when it suspended. It never receives a bmiss notification so it
never tries reassociating.

PR: kern/169084