Revert "SPM: Use flag to indicate NS Agent"

This reverts commit 27f9a49508547320c50056a52a372674c27771eb.

Reason for revert: Breaks dual-core. Will re-submit it as part of agent update later.

Change-Id: I7532570476c8d543297c7f6f2e871413d5e77221
diff --git a/secure_fw/spm/ffm/backend_ipc.c b/secure_fw/spm/ffm/backend_ipc.c
index 69e956d..48c7899 100644
--- a/secure_fw/spm/ffm/backend_ipc.c
+++ b/secure_fw/spm/ffm/backend_ipc.c
@@ -116,7 +116,7 @@
               TO_THREAD_PRIORITY(PARTITION_PRIORITY(p_pldi->flags)));
 
 #if defined(CONFIG_TFM_PSA_API_CROSS_CALL) && !defined(TFM_MULTI_CORE_TOPOLOGY)
-    if (p_pldi->flags & PARTITION_NON_SECURE_AGENT) {
+    if (p_pldi->pid == TFM_SP_NON_SECURE_ID) {
         SPM_THREAD_CONTEXT = &p_pt->ctx_ctrl;
     }
 #endif