net: lan966x: check devm_of_phy_get() for -EDEFER_PROBE

At the moment, if devm_of_phy_get() returns an error the serdes
simply isn't set. While it is bad to ignore an error in general, there
is a particular bug that network isn't working if the serdes driver is
compiled as a module. In that case, devm_of_phy_get() returns
-EDEFER_PROBE and the error is silently ignored.

The serdes is optional, it is not there if the port is using RGMII, in
which case devm_of_phy_get() returns -ENODEV. Rearrange the error
handling so that -ENODEV will be handled but other error codes will
abort the probing.

Fixes: d28d6d2e ("net: lan966x: add port module support")
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220525231239.1307298-1-michael@walle.ccSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/microchip/lan966x/lan966x_main.c

@@ -1120,7 +1120,12 @@ static int lan966x_probe(struct platform_device *pdev)
 		lan966x->ports[p]->fwnode = fwnode_handle_get(portnp);
 
 		serdes = devm_of_phy_get(lan966x->dev, to_of_node(portnp), NULL);
-		if (!IS_ERR(serdes))
+		if (PTR_ERR(serdes) == -ENODEV)
+			serdes = NULL;
+		if (IS_ERR(serdes)) {
+			err = PTR_ERR(serdes);
+			goto cleanup_ports;
+		}
 		lan966x->ports[p]->serdes = serdes;
 
 		lan966x_port_init(lan966x->ports[p]);