igc: Cleanup _get|set_rxnfc ethtool ops

This patch does a trivial change in igc_ethtool_get_rxnfc() and
igc_ethtool_set_rxnfc() to simplify their logic.

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>