drm/ttm: fix return value check

The function returns the number of swapped pages here. Only abort when we get
a negative error code.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210409130113.1459-1-christian.koenig@amd.com

drivers/gpu/drm/ttm/ttm_tt.c

@@ -326,7 +326,7 @@ int ttm_tt_populate(struct ttm_device *bdev,
 	       ttm_dma32_pages_limit) {
 
 		ret = ttm_global_swapout(ctx, GFP_KERNEL);
-		if (ret)
+		if (ret < 0)
 			goto error;
 	}