dm9601: handle corrupt mac address

Some cheap devices ship with dangling EEPROM pins!
They always return invalid address ff:ff:ff:ff:ff:ff.

Inherit the auto-generated address in this case,
so that these products can work with zero configuration.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>