of_mdio: Correct check against CONFIG_OF

CONFIG_OF_MDIO is actually what triggers the build of drivers/of/of_mdio.c, so
providing inline stubs when CONFIG_OF_MDIO=y should be based on that symbol as
well.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>