cfg80211: fix deadlock

When removing an interface with nl80211, cfg80211 will
deadlock in the netdev notifier because we're already
holding rdev->mtx and try to acquire it again to verify
the scan has been done.

This bug was introduced by my patch
"cfg80211: check for and abort dangling scan requests".

To fix this, move the dangling scan request check into
wiphy_unregister(). This will not be able to catch all
cases right away, but if the scan problem happens with
a manual ifdown or so it will be possible to remedy it
by removing the module/device.

Additionally, add comments about the deadlock scenario.

Reported-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Christian Lamparter <chunkeey@web.de>
Tested-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>