SEC Makes Krispy Kreme Probe Formal  NEW YORK (Reuters) - U.S. securities regulators have  ratcheted up their probe of Krispy Kreme Doughnuts Inc.'s  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=KKD.N target=/stocks/quickinfo/fullquote"&gt;KKD.N&lt;/A&gt; franchise buybacks, the company said on Friday,  triggering a sharp drop in its stock.