Sen. Charles Schumer just can’t stay off his cellphone — even at Geraldine Ferraro’s funeral. Following yesterday’s service, Schumer whipped out his phone even before descending the front steps of the Church of St. Vincent Ferrer in Manhattan, The Post’s Jennifer Fermino reports. Ignoring throngs of mourners leaving the church, an oblivious Schumer yakked away before going over to speak to reporters. Schumer’s addiction to his mobile is well documented. In 2009, he called a US Airways flight attendant a “bitch” for ordering him to shut it off before takeoff on a flight from La Guardia to Washington.