stmmac: platform: fix FIXED_PHY support.

On several STi platforms: e.g. stihxxx-b2120 an Ethernet switch is
embedded and connected to the stmmac via RGMII mode. So this is managed
by using the FIXED_PHY. In that case, the support in the platform needs
to be fixed to allow the stmmac to dialog with the switch via fixed-link
by using phy_bus_name property.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c

@@ -160,11 +160,16 @@ static int stmmac_probe_config_dt(struct platform_device *pdev,
 	if (of_property_read_u32(np, "snps,phy-addr", &plat->phy_addr) == 0)
 		dev_warn(&pdev->dev, "snps,phy-addr property is deprecated\n");
 
-	plat->mdio_bus_data = devm_kzalloc(&pdev->dev,
-					   sizeof(struct stmmac_mdio_bus_data),
-					   GFP_KERNEL);
-
-	plat->force_sf_dma_mode = of_property_read_bool(np, "snps,force_sf_dma_mode");
+	if (plat->phy_bus_name)
+		plat->mdio_bus_data = NULL;
+	else
+		plat->mdio_bus_data =
+			devm_kzalloc(&pdev->dev,
+				     sizeof(struct stmmac_mdio_bus_data),
+				     GFP_KERNEL);
+
+	plat->force_sf_dma_mode =
+		of_property_read_bool(np, "snps,force_sf_dma_mode");
 
 	/* Set the maxmtu to a default of JUMBO_LEN in case the
 	 * parameter is not present in the device tree.