mac80211: Don't restart sta-timer if not associated.

I found another crash when deleting lots of virtual stations
in a congested environment.  I think the problem is that
the ieee80211_mlme_notify_scan_completed could call
ieee80211_restart_sta_timer for a stopped interface
that was about to be deleted.

With the following patch I am unable to reproduce the
crash.

Signed-off-by: Ben Greear <greearb@candelatech.com>
[move check, also make the same change in mesh]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>