Fannie Mae is taking some heat from subprime lenders that have challenged the GSE’s claim that about 50 percent of subprime borrowers could qualify for conventional loans if they were offered the lower-cost products. Mitchell Feinstein, chairman of the National Home Equity Mortgage Association, sent a letter earlier this month to Fannie Chairman and CEO Franklin Raines that took the GSE to task for comments made at the annual conference of the Mortgage Bankers Association.…