(Corrects valuation to “over $6 bln” from “over $12 bln” in headline and paragraph 1) Sept 7 (Reuters) - On Holding AG, a shoe firm backed by Swiss tennis star Roger Federer, is targeting a valuation of over $6 billion for an initial public offering in the United States, a regulatory filing showed on Tuesday. (Reporting by Sohini Podder in Bengaluru; Editing by Krishna Chandra Eluri) Our Standards: The Thomson Reuters Trust Principles.