i40e: handle setting administratively set MAC address back to zero

When an administratively set MAC was previously set and should now be
switched back to 00:00:00:00:00:00 the pf_set_mac flag did not get
toggled back to false.
As a result VFs were still treated as if an administratively set MAC was
present.

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>