net: mscc: Fix OF_MDIO config check

When CONFIG_OF_MDIO is set to be a module the code block is not
compiled. Use the IS_ENABLED macro that checks for both built in as
well as module.

Fixes: 4f58e6dc ("net: phy: Cleanup the Edge-Rate feature in Microsemi PHYs.")
Signed-off-by: NDan Murphy <dmurphy@ti.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/phy/mscc/mscc.h

@@ -374,7 +374,7 @@ struct vsc8531_private {
 #endif
 };
 
-#ifdef CONFIG_OF_MDIO
+#if IS_ENABLED(CONFIG_OF_MDIO)
 struct vsc8531_edge_rate_table {
 	u32 vddmac;
 	u32 slowdown[8];

drivers/net/phy/mscc/mscc_main.c

@@ -98,7 +98,7 @@ static const struct vsc85xx_hw_stat vsc8584_hw_stats[] = {
 	},
 };
 
-#ifdef CONFIG_OF_MDIO
+#if IS_ENABLED(CONFIG_OF_MDIO)
 static const struct vsc8531_edge_rate_table edge_table[] = {
 	{MSCC_VDDMAC_3300, { 0, 2,  4,  7, 10, 17, 29, 53} },
 	{MSCC_VDDMAC_2500, { 0, 3,  6, 10, 14, 23, 37, 63} },
@@ -382,7 +382,7 @@ static void vsc85xx_wol_get(struct phy_device *phydev,
 	mutex_unlock(&phydev->lock);
 }
 
-#ifdef CONFIG_OF_MDIO
+#if IS_ENABLED(CONFIG_OF_MDIO)
 static int vsc85xx_edge_rate_magic_get(struct phy_device *phydev)
 {
 	u32 vdd, sd;