i40e: flower: Fix return value for unsupported offload

When filter configuration is not supported, drivers should return
-EOPNOTSUPP so the core can react correctly.

Fixes: 2f4b411a ("i40e: Enable cloud filters via tc-flower")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>