i40e: Retain MAC filters on port VLAN deletion

On port VLAN deletion the list of MAC filters for the virtual function (VF)
VSI were all deleted.  Let's keep them around, they come in handy for keeping
the VF functional.

Change-Id: I335e760392f274dc8b8b40efcb708f65b49d7973
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>