Further OMCI lib-go SW update enhancements

1.) StartSoftwareDownloadRequest coding checks correctly on NumberOfCircuitPacks>1 (even though it might be discussed if that is really required). But if given as 1 it also enforces the first CircuitPack to be defined by the application, even though the original ME EntityInstance is defined to be ONU-G based. I suppose this and further Circuitpack entries are only required for ME EntityInstance 0xFFFF. (ONU-G based by now only working if application inserts NULL-CircuitPack).

2.) StartSoftwareDownloadResponse can only be used with NumberOfInstances=0 (which is sufficient for the OpenOnu Adapter afaik). But if multiple (ME-ID) instance responses should have to be used, then it would be required to export the defined type 'downloadResults', otherwise the application has no access to that.