net:stmmac: Add check if mdiobus is registered in stmmac_mdio_unregister

This patch adds a basic check in stmmac_mdio_unregister to see if mdio
bus registeration for this driver was actually sucessfull or not.

Use case here is, if BSP considers using mdio-gpio bus along with stmmac
driver by passing mdio_bus_data as NULL in platform data.
Call to stmmac_mdio_register with mdio_bus_data as NULL returns 0, which
is a considered sucessfull call form stmmac. Then again when we unload
the driver we just call stmmac_mdio_unregister, this is were the actual
problem is stmmac-mdio code dont really know at this instance of calling
that stmmac_mdio_register was actually successful.

So Adding a check in stmmac_mdio_unregister is always safe.

Without this patch stmmac driver calls stmmac_mdio_register from
stmmac_release which Segfaults as mii bus was never registered at the
first point.

Originally the this bug was found when unloading an stmmac driver
instance which uses mdio-gpio for smi access.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c

@@ -258,6 +258,9 @@ int stmmac_mdio_unregister(struct net_device *ndev)
 {
 	struct stmmac_priv *priv = netdev_priv(ndev);
 
+	if (!priv->mii)
+		return 0;
+
 	mdiobus_unregister(priv->mii);
 	priv->mii->priv = NULL;
 	mdiobus_free(priv->mii);