devlink: fix sb register stub in case devlink is disabled

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: bf797471

 ("devlink: add shared buffer configuration")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>