wifi: cfg80211: clear wdev->cqm_config pointer on free

When we free wdev->cqm_config when unregistering, we also
need to clear out the pointer since the same wdev/netdev
may get re-registered in another network namespace, then
destroyed later, running this code again, which results in
a double-free.

Reported-by:  <syzbot+36218cddfd84b5cc263e@syzkaller.appspotmail.com>
Fixes: 37c20b2e ("wifi: cfg80211: fix cqm_config access race")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20241022161742.7c34b2037726.I121b9cdb7eb180802eafc90b493522950d57ee18@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>