i40e: add ethtool RSS support

Add support for setting the RSS hash table and hash key through ethtool.
This patch incorporates suggestions from Ben Hutchings
<ben.hutchings@codethink.co.uk>.

Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>