net: cpsw: catch of_get_phy_mode failing and propagate error

It's wrong if the device tree doesn't provide a phy-mode property for
the cpsw slaves as it is documented to be required in
Documentation/devicetree/bindings/net/cpsw.txt.

Anyhow it's nice to catch that problem, still more as it used to work
without this property up to commit 388367a5 (drivers: net: cpsw: use
cpsw-phy-sel driver to configure phy mode) which is in v3.13-rc1.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/ti/cpsw.c

@@ -1896,6 +1896,11 @@ static int cpsw_probe_dt(struct cpsw_platform_data *data,
 			memcpy(slave_data->mac_addr, mac_addr, ETH_ALEN);
 
 		slave_data->phy_if = of_get_phy_mode(slave_node);
+		if (slave_data->phy_if < 0) {
+			pr_err("Missing or malformed slave[%d] phy-mode property\n",
+			       i);
+			return slave_data->phy_if;
+		}
 
 		if (data->dual_emac) {
 			if (of_property_read_u32(slave_node, "dual_emac_res_vlan",