igb: Remove useless argument

Given that all callers of igb_update_stats() pass the same two arguments:
(adapter, &adapter->stats64), the second argument can be removed.

Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>