My dog has a yeast infection in his ear. What to do?

Ok, so today I took my dog into the vet and the theytold me he has a yeast infection in his ear. They gave me drops to give him and stuff. But then they mentioned to me I have to COMPLETELY change his diet to this certain expensive brand of food (it only has potato and duck in it) which they are the only one's that sell it, so it doesn't get an infection again because that's the way his breed is (English Setter/Springer Spaniel). Now the reason I'm asking is does it sound right to completely change my dog's diet because of an ear infection? It sounded to me like they just want to make money off expensive dog food..thanks.