[ath] ensure both iv_ampdu_limit and iv_ampdu_rxmax is set.

A recent change enforced the VAP limit as well as the peer limit.
I now need to actually set iv_ampdu_limit or we don't transmit more
than 8K sized aggregates.

This restores the expected (suboptimal, but still much faster) behaviour.

Tested:

• AR9380, STA mode