mac80211: always set the buf_size in AddBA req to 64

Advertising reordering window in ADDBA less than 64 can crash some APs,
an example is LinkSys WRT120N (with FW v1.0.07 build 002 Jun 18 2012).
On the other hand, a driver may need to limit Tx A-MPDU size for its own
reasons, like specific HW limitations.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>