igc: Fix igc_uc_unsync()

In case igc_del_mac_filter() returns error, that error is masked
since the functions always return 0 (success). This patch fixes
igc_uc_unsync() so it returns whatever value igc_del_mac_filter()
returns (0 on success, negative number on error).

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>