igc: Fix returning wrong statistics

'igc_update_stats()' was not updating 'netdev->stats', so the returned
statistics, for example, requested by:

$ ip -s link show dev enp3s0

were not being updated and were always zero.

Fix by returning a set of statistics that are actually being
updated (adapter->stats64).

Fixes: c9a11c23

 ("igc: Add netdev")
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>