drm/nouveau/secboot: fix inconsistent pointer checking

We were returning PTR_ERR() on a NULL pointer, which obviously won't
work. nvkm_engine_ref() will return an error in case something went
wrong.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/secboot/gp102.c

@@ -59,7 +59,8 @@ gp102_run_secure_scrub(struct nvkm_secboot *sb)
 
 	nvkm_debug(subdev, "running VPR scrubber binary on NVDEC...\n");
 
-	if (!(engine = nvkm_engine_ref(&device->nvdec->engine)))
+	engine = nvkm_engine_ref(&device->nvdec->engine);
+	if (IS_ERR(engine))
 		return PTR_ERR(engine);
 	falcon = device->nvdec->falcon;