igc: Fix locking issue when retrieving NFC rules

Access to NFC rules stored in adapter->nfc_rule_list is protect by
adapter->nfc_rule_lock. The functions igc_ethtool_get_nfc_rule()
and igc_ethtool_get_nfc_rules() are missing to hold the lock while
accessing rule objects.

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>