i40e: don't add zero MAC filter

When VFs are created, the MAC address defaults to all zeros, indicating
to the VF driver that it should use a random MAC address. However, the
PF driver was incorrectly adding this zero MAC to the filter table,
along with the VF's randomly generated MAC address.

Check for a good address before adding the default filter. While we're
at it, make the error message a bit more useful.

Change-ID: Ia100947d68140e0f73a19ba755cbffc3e79a8fcf
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>