mac80211: add support for the ADDBA extension element

HE allows peers to negotiate the aggregation fragmentation level to be used
during transmission. The level can be 1-3. The Ext element is added behind
the ADDBA request inside the action frame. The responder will then reply
with the same level or a lower one if the requested one is not supported.
This patch only handles the negotiation part as the ADDBA frames get passed
to the ATH11k firmware, which does the rest of the magic for us aswell as
generating the requests.

Signed-off-by: Shashidhar Lakkavalli <slakkavalli@datto.com>
Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20190729104512.27615-1-john@phrozen.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>