mac80211: delete work timer

The new workqueue changes helped me find this bug
that's been lingering since the changes to the work
processing in mac80211 -- the work timer is never
deleted properly. Do that to avoid having it fire
after all data structures have been freed. It can't
be re-armed because all it will do, if running, is
schedule the work, but that gets flushed later and
won't have anything to do since all work items are
gone by now (by way of interface removal).

Cc: stable@kernel.org [2.6.34+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>