WASHINGTON (Reuters) - White House economic adviser Gary Cohn on Thursday said the administration now expects U.S. economic growth to be “substantially” higher than 3 percent, saying the boost would pay for Republicans’ tax plan.       “We think we can pay for the entire tax cut through growth over the cycle,” Cohn told CNBC. “Our plan was based on a 3 percent GDP growth. We think we can now be substantially above 3 percent GDP growth,” citing the administration’s plans on deregulation and tax reform.  