THE BRONX - A teenager has been charged with assault and menacing yesterday for allegedly beating a Bronx bus driver last month.Steangeli Medina is accused of attacking driver Marlene Bien-Aime for refusing to let her bring her dog on the Bx9 bus.Prosecutors say Medina repeatedly hit the 49-year-old woman in the eye and yanked her hair, causing the driver to fall to the ground. One eyewitness was also injured in the June 21 incident.Riders eventually managed to restrain Medina until police arrived.