Return raw pointer for etl::multi_span::operator->(). (#773)

- using `operator*()` in case getting the current element is changed one day
- using `&` to get the address

Simply returning `p_value` may break in case the internal type of `p_value` would change. This way it is more robust to changes.

include/etl/multi_span.h

@@ -134,15 +134,15 @@ namespace etl
       //*************************************************************************
       pointer operator ->()
       {
-        return *p_value;
+        return &operator*();
       }
 
       //*************************************************************************
       /// -> operator
       //*************************************************************************
       const_pointer operator ->() const
       {
-        return *p_value;
+        return &operator*();
       }
 
       //*************************************************************************