au1000_eth: possible NULL dereference of aup->mii_bus->irq in au1000_probe()

aup->mii_bus->irq allocation may fail, prevent a dereference of NULL.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>