igc: Change return type from igc_disable_nfc_rule()

None of igc_disable_nfc_rule() callers actually check its returning
value. A closer look at why this function would fail shows that the
only situation is when we try to delete an Ethertype or MAC filter that
doesn't exist.

That situation is very unlikely so we can change igc_del_etype_filter()
and igc_del_mac_filter() logic to "if the filter doesn't exist, we are
done", and keep the logic in igc_disable_nfc_rule() callers simple.

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>