drm/amdgpu: Fix incorrect filenames in sysfs comments

This looks like a standard copy/paste mistake. Replace the incorrect
serial_number references with product_name and product_model

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>