cfg80211: don't request disconnect if not connected

Neil Brown reports that with libertas, my recent cfg80211
SME changes in commit ceca7b71
("cfg80211: separate internal SME implementation") broke
libertas suspend because it we now asked it to disconnect
while already disconnected.

The problematic change is in cfg80211_disconnect() as it
previously checked the SME state and now calls the driver
disconnect operation unconditionally.

Fix this by checking if there's a current_bss indicating
a connection, and do nothing if not.
Reported-and-tested-by: Neil Brown <neilb@suse.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/sme.c

@@ -976,21 +976,19 @@ int cfg80211_disconnect(struct cfg80211_registered_device *rdev,
 			struct net_device *dev, u16 reason, bool wextev)
 {
 	struct wireless_dev *wdev = dev->ieee80211_ptr;
-	int err;
+	int err = 0;
 
 	ASSERT_WDEV_LOCK(wdev);
 
 	kfree(wdev->connect_keys);
 	wdev->connect_keys = NULL;
 
-	if (wdev->conn) {
+	if (wdev->conn)
 		err = cfg80211_sme_disconnect(wdev, reason);
-	} else if (!rdev->ops->disconnect) {
+	else if (!rdev->ops->disconnect)
 		cfg80211_mlme_down(rdev, dev);
-		err = 0;
-	} else {
+	else if (wdev->current_bss)
 		err = rdev_disconnect(rdev, dev, reason);
-	}
 
 	return err;
 }