Never an “up” guy, Dave’s had a brutally down year. Jay Leno’s kicking his Big Ass Hams in the ratings. The Oscars didn’t invite him back. CBS canceled a sitcom he produced. His top aides have deserted him. One rumor had his executive producer, Robert (Morty) Morton, fleeing to run “Saturday Night Live.” Dave blames the ailing network for his slump and is promising big changes to combat the “perception that the show sucks.” This week’s L.A. trip might help. And when he’s not taping, he can check out a few retirement homes.