netdevsim: rename a label

err_dl_unregister should unregister the devlink instance.
Looks like renaming it was missed in one of the reshufflings.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>