Fix loading of the driver when built as a module.

Fixes: f160e994 ("net: phy: Add mdio-aspeed")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>