i40e: Add message for unsupported MFP mode

This patch adds a check and message if the device is in
MFP mode as changing RSS input set is not supported in
MFP mode.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>