Initialize diag output callback per instance in NcpBase

This fixes a bug where diag get output callback was not set properly if we used instance with iid > 1, leading to failure output.

src/ncp/ncp_base.cpp

@@ -258,6 +258,9 @@ NcpBase::NcpBase(Instance **aInstances, uint8_t aCount)
 
         OT_ASSERT(i + skipped <= SPINEL_HEADER_IID_MAX);
         mInstances[i + skipped] = aInstances[i];
+#if OPENTHREAD_CONFIG_DIAG_ENABLE
+        otDiagSetOutputCallback(mInstances[i + skipped], &NcpBase::HandleDiagOutput_Jump, this);
+#endif
     }
 }
 #endif // OPENTHREAD_CONFIG_MULTIPAN_RCP_ENABLE  && OPENTHREAD_RADIO
@@ -358,9 +361,6 @@ NcpBase::NcpBase(Instance *aInstance)
     otSrpClientSetCallback(mInstance, HandleSrpClientCallback, this);
 #endif
 #endif // OPENTHREAD_MTD || OPENTHREAD_FTD
-#if OPENTHREAD_CONFIG_DIAG_ENABLE
-    otDiagSetOutputCallback(mInstance, &NcpBase::HandleDiagOutput_Jump, this);
-#endif
     mChangedPropsSet.AddLastStatus(SPINEL_STATUS_RESET_UNKNOWN);
     mUpdateChangedPropsTask.Post();