mac80211: clean up station cleanup timer

We currently run this timer exactly once when
a new mac80211 device is registered, but that
is completely pointless since it will have no
work to do at all. Therefore, remove that and
also simplify some code using the timer.

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