cfg80211: fix a crash in dev lookup on dump commands

IS_ERR and PTR_ERR were called with the wrong pointer, leading to a
crash when cfg80211_get_dev_from_ifindex fails.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/nl80211.c

@@ -224,8 +224,8 @@ static int nl80211_prepare_netdev_dump(struct sk_buff *skb,
 	}
 
 	*rdev = cfg80211_get_dev_from_ifindex(sock_net(skb->sk), ifidx);
-	if (IS_ERR(dev)) {
-		err = PTR_ERR(dev);
+	if (IS_ERR(*rdev)) {
+		err = PTR_ERR(*rdev);
 		goto out_rtnl;
 	}