Universal Studios takes issue with a March 2 Morning Report item that TV star Tom Selleck signed for an eighth season of his CBS "Magnum, P.I." series despite its falling ratings. The studio agrees that the program's ratings have been on the decline in recent years, especially when it ran against the hit "The Cosby Show" on Thursday nights. But, Universal points out, in the year since the series moved to Wednesday night opposite "Dynasty," ratings have rebounded about 20% over 1985-86 season. The series has averaged a 16.4 rating this season, up from its 14.7 rating last year but down from every other year since 1980.