mv643xx_eth: defer probing if Marvell Orion MDIO driver not loaded

When both the Marvell MV643XX ethernet driver and the Orion MDIO driver
are compiled as modules, the ethernet driver may be probed before the
MDIO driver.  Let mv643xx_eth_probe() return EPROBE_DEFER in this case,
i.e. when it cannot find the PHY.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>