net: axienet: fix teardown order of MDIO bus

Since the MDIO is is brought up before the netdev is registered, it
should be torn down after the netdev is removed. Otherwise, PHY accesses
can potentially access freed MDIO bus references and cause a crash.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>