net/devlink: Prepare devlink port functions to fill extack

Prepare devlink port related functions to optionally fill up
the extack information which will be used in subsequent patch by port
function attribute(s).

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>