net: ethernet: fix kernel OOPS when remove davinci_mdio module

davinci mdio device is not unregistered from mdiobus when removing
the module, which causes BUG_ON() when free the device from mdiobus.

Calling mdiobus_unregister() before mdiobus_free() fixes the issue.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>