mac80211: abort CAC in stop_ap()

When a CAC is running and stop_ap is called (e.g. when hostapd is killed
while performing CAC), the CAC must be aborted immediately.
Otherwise ieee80211_stop_ap() will try to stop it when it's too late -
wdev->channel is already NULL and the abort event can not be generated.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>