Currently, the code to tear down BA sessions will
execute after queues are stopped, but attempt to
send frames, so those frames will just get queued,
which isn't intended. Move this code to before to
tear down the sessions properly.

Additionally, after stopping queues, flush the TX
queues in the driver driver to make sure all the
frames went out.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>