My
  commit 77fdaa12
  Author: Johannes Berg <johannes@sipsolutions.net>
  Date:   Tue Jul 7 03:45:17 2009 +0200

      mac80211: rework MLME for multiple authentications

inadvertedly broke WMM because it removed, along with
a bunch of other now useless initialisations, the line
initialising sdata->u.mgd.wmm_last_param_set to -1
which would make it adopt any WMM parameter set. If,
as is usually the case, the AP uses WMM parameter set
sequence number zero, we'd never update it until the
AP changes the sequence number.

Add the missing initialisation back to get the WMM
settings from the AP applied locally.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.31+]
Signed-off-by: John W. Linville <linville@tuxdriver.com>