DataReader: Correct return type of operator=

It's questionable to not return a reference to the instance being
assigned to. It's also quite misleading in terms of expected behavior
relative to everything else. This fixes it to make it consistent with
other classes.

Source/Core/VideoCommon/DataReader.h

@@ -16,10 +16,10 @@ public:
   DataReader() = default;
   DataReader(u8* src, u8* end_) : buffer(src), end(end_) {}
   u8* GetPointer() { return buffer; }
-  u8* operator=(u8* src)
+  DataReader& operator=(u8* src)
   {
     buffer = src;
-    return src;
+    return *this;
   }
 
   size_t size() const { return end - buffer; }