Changes

RAW_NS: SNS Add: handle IPv4/IPv6 element in SNS Ack

When adding a new NS-VC via SNS Add, the SNS Ack should
only contains the IEs "List of IP4/6 elements" if the cause code is
Unknown IP endpoint.

However the libosmocore/ns2 code isn't following this behavior yet.

Change-Id: I18c659e43db79400c1b428ed745267a5e6e59102