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: NDan Murphy <dmurphy@ti.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>