i40e: Enable/Disable PF switch LB on SR-IOV configure changes

The PF VSI was never updated to enable or disable internal switch loopback
when VFs were created or destroyed via the sysfs interface.  Add some
helper functions to take care of that.

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