net: dp83869: 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: 01db923e ("net: phy: dp83869: Add TI dp83869 phy")
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/dp83869.c

@@ -176,7 +176,7 @@ static int dp83869_set_strapped_mode(struct phy_device *phydev)
 	return 0;
 }
 
-#ifdef CONFIG_OF_MDIO
+#if IS_ENABLED(CONFIG_OF_MDIO)
 static int dp83869_of_init(struct phy_device *phydev)
 {
 	struct dp83869_private *dp83869 = phydev->priv;