Fannie Mae Shares Fall on Accounting Woes  WASHINGTON (Reuters) - Shares of U.S. mortgage finance  giant Fannie Mae &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=FNM.N target=/stocks/quickinfo/fullquote"&gt;FNM.N&lt;/A&gt; tumbled on Thursday after the  Securities and Exchange Commission said the company must  restate earnings because of accounting problems.