i40evf: core ethtool functionality

This patch contains the ethtool interface and related functionality.
Since the VF driver is mostly unaware of link, much of that
functionality is unused. The driver implements ethtool hooks for
statistics, driver info, and some basic non-link-related driver
settings.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>