of/mdio: Staticise !CONFIG_OF stubs

The !CONFIG_OF stubs aren't static so if multiple files include the
header with this configuration then the linker will see multiple
definitions of the stubs.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>