igb: Don't give VFs random MAC addresses

If the user has not assigned a MAC address to a VM, then don't give it a
random one. Instead, just give it zeros and let it figure out what to do
with them.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Tested-by: Stefan Assmann <sassmann@redhat.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>