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