1. In a 4- to 5-quart pan, mix masa flour with 1 cup broth until smooth. Stir over high heat until mixture boils, then reduce heat to medium-low and stir until masa is thick, 2 to 3 minutes. Pour onto a flat plate. Spread evenly to make a 3/4-inch-thick cake; let cool.
2. Meanwhile, rinse pan and add remaining broth and garlic. Cover and bring to a boil over high heat.
3. Trim and discard zucchini ends. Cut zucchini into 3/4-inch cubes. Add zucchini to pan; cover and simmer for 5 minutes.
4. Meanwhile, discard husks and silks from corn. Cut corn crosswise into 1-inch-thick wheels. Also cut masa into 3/4-inch chunks. Add corn and masa chunks to broth.
5. Cover pan and return to boiling over high heat; simmer until corn is hot, about 3 minutes.
6. Ladle into bowls. Sprinkle with cilantro. Serve with lime wedges, salt, and pepper to season portions to taste.