WASHINGTON (AP) — Fixed U.S. mortgage rates rose for the sixth straight week, putting the average rate on the 30-year loan just shy of 4 percent.Mortgage buyer Freddie Mac said Thursday that the rate on the 30-year loan increased to 3.98 percent. That’s up from 3.91 percent last week and the highest since April 2012. The average rate was last at 4 percent or higher in March 2012.The rate on the 15-year loan advanced to 3.10 percent from 3.03 percent.