drm/nouveau/secboot: let callers interpret return value of blobs

Since the HS blobs are provided and signed by NVIDIA, we cannot expect
always-consistent behavior. In this case, on GP10x the unload blob may
return 0x1d even though things have run perfectly well. This behavior
has been confirmed by NVIDIA.

So let the callers of the run_blob() hook receive the blob return's
value (a positive integer) and decide what it means. This allows us to
workaround the 0x1d code instead of issuing an error.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>