omci-lib-go .1pMapper create default values

When creating the .1pMapper in most cases the GemiwTp to be set are not yet known, thus requiring the set the InterworkingTP pointers to 'discard'.

For the moment used default value in in the omci-lib-go are '0', I would recommend to change them to 0xFFFF as recommended as discard values in G988.

This is low priority as we can control the values also from outside the omci_lib, but the change in the omci_lib would be more consistent (and robust).