ethernet/intel: Convert fallthrough code comments

Convert all the remaining 'fall through" code comments to the newer
'fallthrough;' keyword.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>