of_mdio: Enable fixed PHY support if driver is a module

The fixed_phy driver doesn't have to be built-in, and it's
important that of_mdio supports it even if it's a module.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>