i40evf: Use net_device_stats from struct net_device

Instead of using a private copy of struct net_device_stats in
struct i40evf_adapter, use stats from struct net_device. Also remove the
now unnecessary .ndo_get_stats function.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>