ixgbe: Fix bogus error message

If the VF sends a MACVLAN request with index of zero then it is not
actually trying to add a filter.  Check the index value and only
indicate that operation is not allowed when the VF is actually trying
to add a filter.

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>