ixgbevf: Adjust to handle unassigned MAC address from PF

If the administrator has not assigned a MAC address to the VF via the
PF then handle it gracefully by generating a temporary MAC address.
This ensures that we always know when we have a random address and
udev won't get upset about it.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>