wireless : use a dedicated workqueue for cfg80211.

This patch moves the works cleanup, scan and events to a cfg80211
dedicated workqueue.

Platform driver like eeepc-laptop ought to use works to rfkill (as
new rfkill does lock in rfkill_unregister and the platform driver is
called from rfkill_switch_all which also lock the same mutex).
This raise a new issue in itself that the work scheduled by the platform
driver to the global worqueue calls wiphy_unregister which flush_work
scan and event works (which thus flush works on the global workqueue inside
a work on the global workqueue) and also put on hold the  wdev_cleanup_work
 (which prevents the dev_put on netdev thus indefinite Usage count error on
wifi device).

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