igc: Refactor igc_ethtool_update_nfc_rule()

Current implementation of igc_ethtool_update_nfc_rule() is a bit
convoluted since it handles too many things: rule lookup, deletion
and addition. This patch breaks it into three functions so we simplify
the code and improve code reuse.

Code related to rule lookup is refactored out to a new function called
igc_get_nfc_rule().

Code related to rule addition is refactored out to a new function called
igc_add_nfc_rule(). This function enables the rule in hardware and adds
it to the adapter's list.

Code related to rule deletion is refactored out to a new function called
igc_del_nfc_rule(). This function disables the rule in hardware, removes
it from adapter's list, and deletes it.

As a byproduct of this refactoring, igc_enable_nfc_rule() and
igc_disable_nfc_rule() are moved to igc_main.c since they are not used
in igc_ethtool.c anymore, and igc_restore_nfc_rules() and igc_nfc_rule_
exit() are moved around to avoid forward declaration.

Also, since this patch already touches igc_ethtool_get_nfc_rule(), it
takes the opportunity to remove the 'match_flags' check. Empty flags
are not allowed to be added so no need to check that.

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>