Good that the cop got charged and fired but the charges should be much more than assault. When a member of the public kicks another, that is assault. When a cop does it, you are under arrest and can't defend yourself. It's an abuse of power. You are effectively a kidnap victim - someone has illegally restrained you and is physically attacking you. You don't know if or when it's going to end. You can't exercise a right of silence and call your lawyer.