Merc Treads dont prevent stuns, they reduce their duration. By seconds, or more commonly fractions of seconds. Garen is tanky enough to last a second or two without attacking, and besides, he basically deals burst anyway.
Garen already has a nice MS steroid in his Q and will often build a FoN. He's doing fine for speed. Tenacity doesn't fit anywhere else into his build. Merc Treads is definitely ideal unless you're playing against a low CC team.