i40e: move ethtool stats boiler plate code to i40e_ethtool_stats.h

Move the boiler plate structures and helper functions we recently
added into their own header file, so that the complete collection is
located together.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>