igb: Fix overwrite of the VF's flags

The vf flags were being overwritten causing the flag that indicates
that the PF has set the VF MAC address to get cleared.  This would
allow the VF to override the MAC address assigned by the Host VMM
using the ip link set command.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>