result: Resolve sign-coversion warnings

The constructor was implicitly using signed->unsigned conversions to
produce 0xFFFFFFFF. We can just specify this explicitly with UINT32_MAX.

src/core/hle/result.h

@@ -183,7 +183,7 @@ class ResultVal {
 public:
     /// Constructs an empty `ResultVal` with the given error code. The code must not be a success
     /// code.
-    ResultVal(ResultCode error_code = ResultCode(-1)) : result_code(error_code) {
+    ResultVal(ResultCode error_code = ResultCode(UINT32_MAX)) : result_code(error_code) {
         ASSERT(error_code.IsError());
     }