Before-the-Bell: OmniVision, Nortel Down  NEW YORK (Reuters) - Shares of OmniVision Technologies Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=OVTI.O target=/stocks/quickinfo/fullquote"&gt;OVTI.O&lt;/A&gt; fell 6.2 percent in premarket trading on Thursday, the  morning after the image sensor designer announced the  resignation of its chief financial officer.