[net80211] correctly (i hope, wow) do a ticks comparison to limit A-MPDU attempts

I was seeing the stack constantly attempt to renegotiate A-MPDU TX
even after 3 failures. My hunch is that the direct ticks comparison
is failing around the ticks wrap-around point.

This failure shouldn't /really/ happen normally, but it turns out being
the IBSS master node on FreeBSD doesn't quite setup 11n right, so
negotiating A-MPDU TX fails.