stmmac: fix oops on rmmod after assigning ip addr

An oops exists in the flow of stmmac_release().
phy_ethtool_get_wol() depends on phydev->drv.
phydev->drv will be null after stmmac_mdio_unreg() completes.

Steps to reproduce on Quark X1000:

1. ifconfig eth0 192.168.0.1
2. rmmod stmmac_pci

To fix this stmmac_mdio_unreg() should be run after unregister_netdev().

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Reported-by: Dan O'Donovan <dan.odonovan@emutex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>