nl80211: allow CAC only if no operation is going on

A CAC should fail if it is triggered while the interface is already
running.
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -5591,6 +5591,9 @@ static int nl80211_start_radar_detection(struct sk_buff *skb,
 	if (err)
 		return err;
 
+	if (netif_carrier_ok(dev))
+		return -EBUSY;
+
 	if (wdev->cac_started)
 		return -EBUSY;