mac80211: cancel restart_work explicitly instead of depending on flush_scheduled_work()

iee80211_hw->restart_work is the only work which uses the system
workqueue.  Instead of calling flush_scheduled_work() during
iee80211_exit(), cancel the work during unregistration.

This is to prepare for the deprecation and removal of
flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>