mac80211: fix powersave

Some of the recent MLME rework I did broke powersave
because the ps_sdata isn't assigned at the right time,
and the work item wasn't removed from the list before
calling ieee80211_recalc_ps(). To be more specific,
this broke the case where you'd enabled PS before
associating, either automatically or with iwconfig.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>