This is now implemented by mac80211 (commit below).
mac80211 will flush/drop the frames on the queues before
suspending / disconnecting.
It will then send the deauth and wait until the queues are
empty.

commit 3b24f4c6
Author: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Date:   Wed Jan 7 15:42:39 2015 +0200

    mac80211: let flush() drop packets when possible

This reverts commit 4e6c48e0.