netdev: always register for single CQM threshold

If the extended feature for CQM levels was not supported no CQM
registration would happen, not even for a single level. This
caused IWD to completely lose the ability to roam since it would
only get notified when the kernel was disconnecting, around -90
dBm, not giving IWD enough time to roam.

Instead if the extended feature is not supported we can still
register for the event, just without multiple signal levels.

src/netdev.c

@@ -4427,9 +4427,10 @@ static int netdev_cqm_rssi_update(struct netdev *netdev)
 
 	if (!wiphy_has_ext_feature(netdev->wiphy,
 					NL80211_EXT_FEATURE_CQM_RSSI_LIST))
-		return 0;
-
-	msg = netdev_build_cmd_cqm_rssi_update(netdev, netdev->rssi_levels,
+		msg = netdev_build_cmd_cqm_rssi_update(netdev, NULL, 0);
+	else
+		msg = netdev_build_cmd_cqm_rssi_update(netdev,
+						netdev->rssi_levels,
 						netdev->rssi_levels_num);
 	if (!msg)
 		return -EINVAL;