ixgbe: Add GET_RSS_KEY command to VF-PF channel commands set

For 82599 and x540 VFs and PF share the same RSS Key. Therefore we will
return the same RSS key for all VFs.

Support for other devices will be added later.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>