ixgbe: Return success or failure on VF MAC filter set

When setting a MAC filter for the VF the function should return a success
or failure code, not the index of the new filter.  It causes spurious NACK
returns to the VF driver.

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