mac80211: stop timers before canceling work items

Re-order the quiesce code so that timers are always
stopped before work-items are flushed. This was not
the problem I saw, but I think it may still be more
correct.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>