drm/nouveau: make sure ret is initialized in nouveau_ttm_io_mem_reserve

This wasn't initialized for pre NV50 hardware.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-and-Tested-by: Mark Hounschell <markh@compro.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/series/84298/

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -1215,8 +1215,8 @@ retry:
 			}
 
 			reg->bus.offset = handle;
-			ret = 0;
 		}
+		ret = 0;
 		break;
 	default:
 		ret = -EINVAL;