net: bcmgenet: Generate a random MAC if none is valid

Instead of having a hard failure and stopping the driver's probe
routine, generate a random Ethernet MAC address to keep going.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>