Fix for build with NPU disabled

* if NPU timing adapter enabled is defined and target subsystem is sse-310
* set timing adapter to off

Change-Id: I53ca0bd8a56e3ede009b745dba7aa8dbee7a81dc
diff --git a/scripts/cmake/platforms/mps3/build_configuration.cmake b/scripts/cmake/platforms/mps3/build_configuration.cmake
index 6f91f8c..7bed48d 100644
--- a/scripts/cmake/platforms/mps3/build_configuration.cmake
+++ b/scripts/cmake/platforms/mps3/build_configuration.cmake
@@ -34,7 +34,7 @@
     # Arm Corstone-310's timing adapter behaviour is very different to Arm Corstone-300 and cannot
     # be used for bandwidth/latency related performance sweeps for the Arm Ethos-U NPU. Read
     # docs/sections/timing_adapters.md for more details.
-    if ((TARGET_SUBSYSTEM STREQUAL sse-310) AND ${ETHOS_U_NPU_TIMING_ADAPTER_ENABLED})
+    if ((TARGET_SUBSYSTEM STREQUAL "sse-310") AND (DEFINED ETHOS_U_NPU_TIMING_ADAPTER_ENABLED))
         message(STATUS "Timing adapter will NOT be used for target subsystem ${TARGET_SUBSYSTEM}")
         set(ETHOS_U_NPU_TIMING_ADAPTER_ENABLED OFF CACHE BOOL "Use of TA" FORCE)
     endif()