By Brad Haynes and Guillermo Parra-Bernal SAO PAULO (Reuters) – Nextel operator NII Holdings Inc is hanging on to its Brazilian unit even though the welterweight mobile carrier has filed for bankruptcy protection in the United States and sold off other Latin American operations. To pull off a turnaround in Brazil, the carrier is counting on regulations and broadcast spectrum that, because of its razor-thin market share, could give it competitive advantages unavailable to major rivals. …