cfg80211: fix NETDEV_UNREGISTER notifier

It's possible to get the NETDEV_UNREGISTER callback multiple
times (see net/core/dev.c:netdev_wait_allrefs) and this will
completely mess up our cleanup code. To avoid that, clean up
only when the interface is still on the wiphy interface list
from which it's removed on the first NETDEV_UNREGISTER call.

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