gprs_subscriber: gsup_auth_error: reject MS in all cases

There is no way to recover from "PROTOCOL_ERRORS".
As long the error_cause is not set, the
SGSN won't send out a GMM Request Reject.

Fixes: TTCN: SGGN_Tests.TC_attach_auth_sai_reject
Change-Id: Iefe8f05686ef4acac721f3c0672910704f3b0ff8
1 file changed