In his latest turn as a TV spokesman for an FBI initiative against insider trading, actor Michael Douglas is doing penance for playing his best-known film role all too well. In the 1987 drama “Wall Street,’’ the corporate raider Gordon Gekko famously intones that “greed, for lack of a better word, is good.’’ Some viewers drew the wrong conclusion. The bureau’s new ad, which was released last week and is intended for financial news networks, aims to clarify things.