igc: Add 'igc_ethtool_' prefix to functions in igc_ethtool.c

This patch adds the prefix 'igc_ethtool_' to all functions defined in
igc_ethtool.c so they align with the name convention already followed by
other parts of the driver (e.g. igc_tsn, igc_ptp). Also, this avoids
some name clashing with functions added to igc_main.c by upcoming
patches in this series. No functionality is changed by this patch, just
function renaming.

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>