igc: Refactor igc_del_mac_filter()

This patch does a code refactoring in igc_del_mac_filter() so it uses
the new helper igc_find_mac_filter() and improves the comment about the
special handling when deleting the default filter.

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>