macsonic: fix crash on PowerBook 520

No-one seems to know where the PowerBook 500 series store their ethernet
MAC addresses. So, rather than crash, use a MAC address from the SONIC
CAM. Failing that, generate a random one.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>