fix(sctp): simple fix for SCTP NG deconnection / shutdown

Should fix this Issue

As written in the fix:

• sctp_handle_com_down function returns 0
• it was sent back to upper loop
• where we check if sctp_read_from_socket method returns SCTP_RC_DISCONNECT.
• which is equal to 1 (src/sctp/sctp_server.hpp)

The fix is quite simple and I've tested w/ 3 gnbsim instances connecting and disconnecting multiple times.

docker stats on the amf container remains in close to 0% CPU Usage