igc: Early return in igc_get_ethtool_nfc_entry()

This patch re-writes the second half of igc_ethtool_get_nfc_entry() to
follow the 'return early' pattern seen in other parts of the driver and
removes some duplicate comments.

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>