cleanup USBAudioObject

[Wouter1]

Could use better error reporting and less code duplication

Eg use this code once instead of everywhere again

    EMUUSBAudioStreamObject*    stream = GetStreamObject(interfaceNum, altInterfaceNum);
    if (!stream) {
        IOLog("Bug? EMUUSBAudioConfigObject::GetEndpointPollInterval stream not opening");
        return 1; // can we cancel the whole thing?
    }

similar for getting the endpoint.