Representative John D. Dingell has ridden to the rescue of U. S. television networks. They're now likely to get more control over foreign syndication of reruns. Since spring, the Michigan Democrat has been grumbling about Federal Communications Commission rules that allowed the nets into the syndication business. Dingell, whose Energy & Commerce Committee oversees the FCC, feels that the rules give foreign-owned studios an advantage over the U. S.-owned nets. An FCC source says the agency will soon issue new rules putting the networks and studios on a more even footing.