stmmac: Move the mdio_register/_unregister in probe/remove

This patch moves the mdio_register/_unregister in probe/remove
functions and this also is required when hibernation on disk
is done.

Signed-off-by: Francesco Virlinzi <francesco.virlinzi@st,com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st,com>
Signed-off-by: David S. Miller <davem@davemloft.net>