Sprint-Nextel Deal Talk Sparks Concern  NEW YORK (Reuters) - The prospect of a deal between Sprint  and Nextel Communications sparked concerns on Friday about a  shrinking U.S. market for mobile network equipment, sending  shares of Nextel's key supplier Motorola Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MOT.N target=/stocks/quickinfo/fullquote"&gt;MOT.N&lt;/A&gt; down  almost 8 percent.