Heat oil in a medium saucepan over low heat. Add cumin and coriander seeds cook, stirring often, until seeds begin to pop. Add curry powder, dry mustard, garlic and ginger and cook until fragrant. Add chile, tomatoes,broth and salt. Bring to a boil. Add sweet potato and cook and return to a boil. Cook over low heat until sweet potato is tender. Serve with lime wedges.