mac80211: fix multi-use timer

We have, sometimes, multiple things that want to
run but don't have their own timer. Introduce a
new function to mac80211's mlme run_again() that
makes sure that the timer will run again at the
_first_ needed time, use that function and also
properly reprogram the timer once it fired.

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