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: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/dev.c b/net/core/dev.c
index bfa9a6a..7593393 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2859,7 +2859,7 @@
 	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);