By Courtney Sherwood PORTLAND, Ore., Sept 22 (Reuters) - A former U.S. attorney for Oregon has endorsed an upcoming referendum to legalize recreational marijuana in the state and tax and regulate its sales, saying on Monday that laws banning the drug have failed to curb its use. Kris Olson, who served as Oregon's chief federal prosecutor during the Clinton administration, is the latest public figure to endorse the ballot measure, which if approved by voters in November would allow recreational use of marijuana for those older than 21.