[attribute storage] revert attempt to use constexpr to skip compiling…

… code

Turns out that while this works well on current compilers, it does not with many embedded toolchains.

- idea was that an if testing an always false constexpr bool should
  skip *compiling* its body.

- we need that because that body refers to a struct field that is non-existing
  in the case when the if condition is constexpr false

- note that we can't use a real >=C++17 constexpr if, because in the
  CHIP_DEVICE_CONFIG_DYNAMIC_ENDPOINT_COUNT>0 case, the condition cannot be a constexpr

Bottom line: there seems no way around the uglier #ifdef-in-middle-of-condition.

src/app/util/attribute-storage.cpp

@@ -705,8 +705,6 @@ Status emAfReadOrWriteAttribute(const EmberAfAttributeSearchRecord * attRecord,
                 // Endpoint processing starts here, so reset the offset.
                 attributeStorageOffset = 0;
             }
-#else
-            constexpr bool hasDynamicAttributeStorage = false;
 #endif
 
             for (clusterIndex = 0; clusterIndex < endpointType->clusterCount; clusterIndex++)
@@ -736,10 +734,12 @@ Status emAfReadOrWriteAttribute(const EmberAfAttributeSearchRecord * attRecord,
                                 {
                                     attributeLocation = singletonAttributeLocation(am);
                                 }
+#if CHIP_DEVICE_CONFIG_DYNAMIC_ENDPOINT_COUNT > 0
                                 else if (hasDynamicAttributeStorage)
                                 {
                                     attributeLocation = emAfEndpoints[ep].dynamicAttributeStorage.data();
                                 }
+#endif
                                 else
                                 {
                                     attributeLocation = attributeData;
@@ -784,7 +784,11 @@ Status emAfReadOrWriteAttribute(const EmberAfAttributeSearchRecord * attRecord,
 
                                 // Internal storage is only supported for fixed endpoints
                                 // and dynamic ones with dynamicAttributeStorage assigned.
-                                if (!isDynamicEndpoint || hasDynamicAttributeStorage)
+                                if (!isDynamicEndpoint
+#if CHIP_DEVICE_CONFIG_DYNAMIC_ENDPOINT_COUNT > 0
+                                    || hasDynamicAttributeStorage
+#endif
+                                )
                                 {
                                     return typeSensitiveMemCopy(attRecord->clusterId, dst, src, am, write, readLength);
                                 }