mac80211: AMPDU handling for rekeys with Extended Key ID

Extended Key ID allows A-MPDU sessions while rekeying as long as each
A-MPDU aggregates only MPDUs with one keyid together.

Drivers able to segregate MPDUs accordingly can tell mac80211 to not
stop A-MPDU sessions when rekeying by setting the new flag
IEEE80211_HW_AMPDU_KEYBORDER_SUPPORT.

Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Link: https://lore.kernel.org/r/20190629195015.19680-3-alexander@wetzel-home.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>