A yeast ear infection is THE most common thing a Labrador can get in his or her life. There are probably 100 different topical ear cleaners and medications that can be safely used. Why go to an oral medication that has some potential serious side effects?
Diflucan is a drug that should only be prescribed by a veterinarian, and not for a run of the mill yeast infection in the ears.
Consult your veterinarian if your current ear treatment is not working. There are many different treatments out there, most involve some time and work on the ears.