drm/rockchip: Fix type promotion bug in rockchip_gem_iommu_map()

The "ret" variable is declared as ssize_t and it can hold negative error
codes but the "rk_obj->base.size" variable is type size_t.  This means
that when we compare them, they are both type promoted to size_t and the
negative error code becomes a high unsigned value and is treated as
success.  Add a cast to fix this.

Fixes: 38f993b7 ("drm/rockchip: Do not use DMA mapping API if attached to IOMMU domain")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/2bfa28b5-145d-4b9e-a18a-98819dd686ce@moroto.mountain

drivers/gpu/drm/rockchip/rockchip_drm_gem.c

@@ -40,7 +40,7 @@ static int rockchip_gem_iommu_map(struct rockchip_gem_object *rk_obj)
 
 	ret = iommu_map_sgtable(private->domain, rk_obj->dma_addr, rk_obj->sgt,
 				prot);
-	if (ret < rk_obj->base.size) {
+	if (ret < (ssize_t)rk_obj->base.size) {
 		DRM_ERROR("failed to map buffer: size=%zd request_size=%zd\n",
 			  ret, rk_obj->base.size);
 		ret = -ENOMEM;