net: marvell: 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: cf41a51db8 ("of/phylib: Use device tree properties to initialize Marvell PHYs.")
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/marvell.c

@@ -429,7 +429,7 @@ static int m88e1101_config_aneg(struct phy_device *phydev)
 	return marvell_config_aneg(phydev);
 }
 
-#ifdef CONFIG_OF_MDIO
+#if IS_ENABLED(CONFIG_OF_MDIO)
 /* Set and/or override some configuration registers based on the
  * marvell,reg-init property stored in the of_node for the phydev.
  *