NEW YORK — A lawyer for former Lt. Gov. Brian Benjamin called the corruption case against his client “flimsy” Monday as the feds revealed they’d seized his cellphone and issued hundreds of subpoenas. Benjamin, 45, pleaded not guilty last week to steering $50,000 in state funds to a nonprofit, Friends of Public School Harlem, in exchange for fraudulent contributions to his unsuccessful bid for city comptroller. He was released on $250,000 bail Tuesday and announced his resignation from office. Facing corruption charges that carry a maximum of 55 years in prison, Benjamin has lawyered up. He hir…