net/ethtool/ioctl: return -EOPNOTSUPP if we have no phy stats

It's not very useful to copy back an empty ethtool_stats struct and
return 0 if we didn't actually have any stats. This also allows for
further simplification of this function in the future commits.
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethtool/ioctl.c

@@ -2099,7 +2099,8 @@ static int ethtool_get_phy_stats(struct net_device *dev, void __user *useraddr)
 		return n_stats;
 	if (n_stats > S32_MAX / sizeof(u64))
 		return -ENOMEM;
-	WARN_ON_ONCE(!n_stats);
+	if (WARN_ON_ONCE(!n_stats))
+		return -EOPNOTSUPP;
 
 	if (copy_from_user(&stats, useraddr, sizeof(stats)))
 		return -EFAULT;