mac80211_hwsim: fix unregistration

If you rmmod the module while associated, frames might
be transmitted during unregistration -- which will crash
if the hwsim%d interface is unregistered first, so only
do that after all the virtual wiphys are gone.

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