i40evf: support ethtool RSS options

Add support for getting and setting RSS hashing options, RSS LUT
entries, and getting the number of device channels from ethtool. Because
the LUT is so small for the VFs, we just read and write the device
registers directly instead of maintaining a shadow copy.

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