drm/amdgpu: refine amdgpu_fru_get_product_info

1. eliminate potential array index out of bounds.
2. return meaningful value for failure.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Jack Gui <Jack.Gui@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>