net: macb: NULL out phydev after removing mdio bus

To ensure the dev->phydev pointer is not used after becoming invalid in
mdiobus_unregister, set it to NULL. This happens when removing the macb
driver without first taking its interface down, since unregister_netdev
will end up calling macb_close.

Signed-off-by: Xander Huff <xander.huff@ni.com>
Signed-off-by: Nathan Sullivan <nathan.sullivan@ni.com>
Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Reviewed-by: Moritz Fischer <moritz.fischer@ettus.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>