net80211: add IEEE80211_CONF_AMPDU_OFFLOAD for AMPDU[-TX] offload

Drivers will set IEEE80211_FEXT_AMPDU_OFFLOAD to indicate to
net80211 that the driver or the firmware will handle AMPDU[-TX]
entirely on their own and net80211 should not do anything.

Following the IEEE80211_CONF_FRAG_OFFLOAD() example add a
IEEE80211_CONF_AMPDU_OFFLOAD() check in net80211 to handle the
condition.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D49829