A former Paterson patrolman charged with using his flashlight to beat a suspect while making an arrest took the witness stand on Friday to defend himself against brutality charges that could send him to prison. Spencer Finch, a 17-year-veteran of the Paterson Police Department, is accused of hitting suspect Brandon Cosby 14 times with his flashlight. Finch took the witness stand in Superior Court in Passaic County on Friday trying to fend off charges of using excessive force to subdue Cosby, who was arrested and charged with making terroristic threats against the mother of his two children. Th…