igc: Refactor igc_ethtool_add_nfc_rule()

Current implementation of igc_ethtool_add_nfc_rule() is quite long and a
bit convoluted so this patch does a code refactoring to improve the
code.

Code related to NFC rule object initialization is refactored out to the
local helper function igc_ethtool_init_nfc_rule(). Likewise, code
related to NFC rule validation is refactored out to another local
helper, igc_ethtool_is_nfc_rule_valid().

RX_CLS_FLOW_DISC check is removed since it is redundant. The macro is
defined as the max value fsp->ring_cookie can have, so checking if
fsp->ring_cookie >= adapter->num_rx_queues is already sufficient.

Finally, some log messages are improved or added, and obvious comments
are removed.

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>