net/core: revert "net: fix __netdev_update_features return.." and add comment

This reverts commit 00ee5927 ("net: fix __netdev_update_features return
on ndo_set_features failure")
and adds a comment explaining why it's okay to return a value other than
0 upon error. Some drivers might actually change flags and return an
error so it's better to fire a spurious notification rather than miss
these.

CC: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -6436,7 +6436,10 @@ int __netdev_update_features(struct net_device *dev)
 		netdev_err(dev,
 			"set_features() failed (%d); wanted %pNF, left %pNF\n",
 			err, &features, &dev->features);
-		return 0;
+		/* return non-0 since some features might have changed and
+		 * it's better to fire a spurious notification than miss it
+		 */
+		return -1;
 	}
 
 sync_lower: