i40evf: set IFF_UNICAST_FLT flag for the VF

Set IFF_UNICAST_FLT flag for the VF to prevent it from entering
promiscuous mode when macvlan is added to the VF.

Signed-off-by: Lihong Yang <lihong.yang@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>