net: fix dev_set_promiscuity() breakage

Commit dad9b335 (netdevice: Fix promiscuity and allmulti overflow) broke
dev_set_promiscuity() by returning on success without reprogramming the
device.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2859,7 +2859,7 @@ int dev_set_promiscuity(struct net_device *dev, int inc)
 	int err;
 
 	err = __dev_set_promiscuity(dev, inc);
-	if (!err)
+	if (err < 0)
 		return err;
 	if (dev->flags != old_flags)
 		dev_set_rx_mode(dev);