[ath] obey the peer A-MPDU density and max-size.
- Obey the peer A-MPDU density if it's larger than the currently configured one.
- Pay attention to the peer A-MPDU max-size and don't assume we can transmit a full A-MPDU (64k!) if the peer announces smaller values.
Relnotes: ath(4): Fix A-MPDU transmit; obey A-MPDU density and max size.