[rocm] Fix wrong attention log (#18764)

Signed-off-by: Felix Marty <felmarty@amd.com>

vllm/platforms/rocm.py

@@ -194,8 +194,9 @@ class RocmPlatform(Platform):
                     f" The selected backend, {selected_backend.name},"
                     f"is not MLA type while requested for MLA backend.")
 
-        selected_backend = (_Backend.ROCM_FLASH if selected_backend
-                            == _Backend.FLASH_ATTN else selected_backend)
+        if selected_backend is None or selected_backend == _Backend.FLASH_ATTN:
+            selected_backend = _Backend.ROCM_FLASH
+
         if envs.VLLM_USE_V1:
             logger.info("Using Triton Attention backend on V1 engine.")
             return ("vllm.v1.attention.backends."