Therefore the chain of logic, "I can't do calculus; other human beings can; therefore, I am disabled" must also be valid.
The human body is intended to be able to hear. If it weren't, we wouldn't born with ears. If a deaf person's ears do not allow him/her to hear, the ears are clearly not functioning properly. Hearing is not a learned skill. It is a biological function of the body.
"Disability" is not a moral or emotional judgment. It's simply a clinical definition. If any part of your body doesn't work as intended, that part of your body is disabled.
Your calculus analogy makes no sense.