ep93xx-eth: check for zero MAC address on probe, not on device open

If we happen to have registered the driver without passing
a MAC address, we will print a zero MAC address and register
the interface with this invalid address, this is confusin. This
patch moves the checking of a valid ethernet address and the
generation of a random one down from the open function to
the probe function.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>