net/mlx4_en: Handle unassigned VF MAC address correctly

When a VF sense they didn't get MAC address, use random one. This will
address the case of administrator not assigning MAC to the VF through
the PF OS APIs and keep udev happy.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>