rtwn: use ieee80211_ht_get_node_ampdu_density(), fix programming MAX_AGG

• use ieee80211_ht_get_node_ampdu_density() now instead of the vap->iv_ampdu_density, so the correct density is used in AP/IBSS/mesh modes.

• MAX_AGG controls how many frames are to be sent in an A-MPDU. It maps to ((MAX_AGG * 2) + 1) == npackets. 0x1f (31) means 64 packets. So, instead of hard-coding 0x1f, use the negotiated block-ack window size.

Differential Revision: https://reviews.freebsd.org/D47766