* Trading volatile ahead of weekend Greek elections * Spanish bond yields rise, increasing anxiety * Michael Kors rallies after results, outlook * Dow up 0.15 pct, S&P flat, Nasdaq down 0.1 pct By Ryan Vlastelica NEW YORK, June 12 (Reuters) - U.S. stocks traded flat on Tuesday as yields on Spain's 10-year bond hit a euro-era high, pointing to continued stress in the nation's debt markets shortly after an announced European Union bailout. While a $125 billion aid packag