mac80211_hwsim: add workqueue to wait for deferred radio deletion on mod unload

When closing multiple wmediumd instances with many radios and try to
unload the  mac80211_hwsim module, it may happen that the work items live
longer than the module. To wait especially for this deletion work items,
add a work queue, otherwise flush_scheduled_work would be necessary.

Signed-off-by: Benjamin Beichler <benjamin.beichler@uni-rostock.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>