drm/amdgpu: refine sdma4 ras_data_cb

simplify code logic and refine return value

v2: remove unused error source code

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>