net: stmmac: Fix error path after register_netdev move

Commit 57016590 ("net: stmmac: Fix race between stmmac_drv_probe and
stmmac_open") re-ordered how the MDIO bus registration and the network
device are registered, but missed to unwind the MDIO bus registration in
case we fail to register the network device.

Fixes: 57016590 ("net: stmmac: Fix race between stmmac_drv_probe and stmmac_open")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Kweh, Hock Leong <hock.leong.kweh@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>