ixgb: Omit private ndo_get_stats function

ixgb_get_stats() just returns dev->stats so we can leave it
out altogether and let dev_get_stats() do the job.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>