net-next: ax88796: Attach MII bus only when open

Call ax_mii_init in ax_open(), and unregister/remove mdiobus resources
in ax_close().

This is needed to be able to unload the module, as the module is busy
while the MII bus is attached.

Signed-off-by: Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>