drm/amdgpu: Resolve bug in UMC 6.7 error offset calculation

Use correct channel and instance values

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>