iwlegacy: cleanup end of il_send_add_sta()

This code causes a static checker warning because we check for
"if (ret == 0)" but we have already had verified that was true.  Clean
it up a little.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>