igc: Change igc_add_mac_filter() returning value

In case of success, igc_add_mac_filter() returns the index in
adapter->mac_table where the requested filter was added. This
information, however, is not used by any caller of that function.
In fact, callers have extra code just to handle this returning
index as 0 (success).

So this patch changes the function to return 0 on success instead,
and cleans up the extra code.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>