By Jonathan Spicer ISTANBUL (Reuters) -The U.S. ambassador to Turkey said he anticipates that President Tayyip Erdogan will give a final sign-off on Sweden’s NATO membership within days, triggering rapid steps toward U.S. Congress endorsing a sale of F-16 fighter jets to Ankara. In an interview on Thursday, Ambassador Jeff Flake said that once the formal ratification document is received in Washington, the U.S. State Department will immediately send Congress notification of the $20 billion F-16s sale. Turkey’s parliament ratified Sweden’s NATO membership bid on Tuesday, clearing a major hurdle…