omci-lib-go: Extended message set download decode/serialize issues

During work on JIRA-4934 (AES-128 issue), the decode and serialization of several extended message set software download request/responses was found to be incorrect.

The issue deals with how the request/responses are formatted if only a single ONU is targeted or if parallel downloads to several ONUs in a chassis is the destination. The single ONU format for the ones in questions do not contain the 'number of instances' or related attribute and any additional data. The bug is that I was including it and should not.

The corrected code now serializes the requests/responses correctly and decodes them as well. To insure better interoperability, the decode is relaxed and if only a 'single' ONU is targeted but encoded as in the parallel case, omci-lib-go will still decode it correctly. This should help if any ONU vendors made the same mistake I did. It will also allow for BBSIM to perhaps use an older omci-lib-go library with the bug and not break any tests.