net/mlx4_core: Don't give VFs MAC addresses which are derived from the PF MAC

If the user has not assigned a MAC address to a VM, then don't give it MAC which
is based on the PF one. The current derivation scheme is wrong and leads to VM
MAC collisions when the number of cards/hypervisors becomes big enough.

Instead, just give it zeros and let them figure out what to do with that.

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