Changes

support RAB Assignment in cn_ranap_rx_*_msg_co()

Allow decoding all Assignment Request/Response messages.

A RAB Assignment request is
- connection oriented
- initiatingMessage
- procedureCode == RAB_Assignment

A RAB Assignment response is
- connection oriented
- successfulOutcome or unsuccessfulOutcome
- procedureCode == RAB_Assignment

Add decoding the request to cn_ranap_rx_initiating_msg_co().
Add decoding the unsuccessful response to
cn_ranap_rx_unsuccessful_msg_co();

Decoding the successful response is already present in
cn_ranap_rx_successful_msg_co().

osmo-gsm-shark uses this to connect a voice call's signalling with RTP.

Change-Id: Ifec566a98cb6141d27b9e5e33d5a78f8b1530658