By Jonathan Stempel NEW YORK (Reuters) -The U.S. Department of Labor has been sued by former Morgan Stanley financial advisers for allegedly issuing an illegal advisory opinion sought by the Wall Street bank that could scuttle hundreds of arbitration claims against the bank. In a complaint filed on Tuesday in Manhattan federal court, the plaintiffs said the Labor Department's finding that Morgan Stanley's deferred incentive compensation plan wasn't an employee benefit pension plan under the federal law k ...