net: dsa: don't print non-fatal MTU error if not supported

Commit 72579e14 ("net: dsa: don't fail to probe if we couldn't set
the MTU") changed, for some reason, the "err && err != -EOPNOTSUPP"
check into a simple "err". This causes the MTU warning to be printed
even for drivers that don't have the MTU operations implemented.
Fix that.
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/slave.c

@@ -1784,7 +1784,7 @@ int dsa_slave_create(struct dsa_port *port)
 	rtnl_lock();
 	ret = dsa_slave_change_mtu(slave_dev, ETH_DATA_LEN);
 	rtnl_unlock();
-	if (ret)
+	if (ret && ret != -EOPNOTSUPP)
 		dev_warn(ds->dev, "nonfatal error %d setting MTU on port %d\n",
 			 ret, port->index);