CircularBuffer: volatile specifier removal

Volatile specifier in this case it not required as we currently have all accesses
to the buffer protected by critical section. This shall optimize accesses in
some cases to the buffer.

Fixes #7702

platform/CircularBuffer.h

@@ -194,9 +194,9 @@ public:
 
 private:
     T _pool[BufferSize];
-    volatile CounterType _head;
-    volatile CounterType _tail;
-    volatile bool _full;
+    CounterType _head;
+    CounterType _tail;
+    bool _full;
 };
 
 /**@}*/