add missing condition for bf16 (#2502)

Without this DataType = unknown -
``` sh
Run Flatmm kernel with DataType = unknown M =1280 N =16384 K =1024 StrideA =1024 StrideB =1024 StrideC =16384 : 0.228837 ms, 187.687 TFlops, 341.374 GB/s,
```

after this change
```sh
Run Flatmm kernel with DataType = bf16 M =1280 N =16384 K =1024 StrideA =1024 StrideB =1024 StrideC =16384 : 0.227029 ms, 189.181 TFlops, 344.092 GB/s,
```

example/ck_tile/18_flatmm/run_flatmm_example.inc

@@ -18,6 +18,10 @@ constexpr const char* DataTypeToString()
     {
         return "bf8";
     }
+    else if  constexpr(std::is_same_v<T, ck_tile::bf16_t>)
+    {
+        return "bf16";
+    }
     else
     {
         return "unknown";