net: marvell: Fix OF_MDIO config check

net: marvell: Fix OF_MDIO config check

[ Upstream commit 5cd119d9a05f1c1a08778a7305b4ca0f16bc1e20 ]

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: cf41a51db8985 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>