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: cf41a51d ("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>