net: dsa: return early if there is no master

It doesn't make sense to have DSA without a master port. Error out early
if there is no master port.

Fixes: fc054d5 ("net: Introduce DSA class for Ethernet switches")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>

net/dsa-uclass.c

@@ -280,6 +280,10 @@ static int dsa_port_probe(struct udevice *pdev)
 	if (!port_pdata->phy)
 		return -ENODEV;
 
+	master = dsa_get_master(dev);
+	if (!master)
+		return -ENODEV;
+
 	/*
 	 * Inherit port's hwaddr from the DSA master, unless the port already
 	 * has a unique MAC address specified in the environment.
@@ -288,10 +292,6 @@ static int dsa_port_probe(struct udevice *pdev)
 	if (!is_zero_ethaddr(env_enetaddr))
 		return 0;
 
-	master = dsa_get_master(dev);
-	if (!master)
-		return 0;
-
 	master_pdata = dev_get_plat(master);
 	eth_pdata = dev_get_plat(pdev);
 	memcpy(eth_pdata->enetaddr, master_pdata->enetaddr, ARP_HLEN);