wifi: cfg80211: clear wdev->cqm_config pointer on free
In the Linux kernel, the following vulnerability has been resolved:

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.