drm/msm: Fix error return checking

The error checks on ret for a negative error return always fails because
the return value of iommu_map_sg() is unsigned and can never be negative.

Detected with Coccinelle:
drivers/gpu/drm/msm/msm_iommu.c:69:9-12: WARNING: Unsigned expression
compared with zero: ret < 0

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
CC: Rob Clark <robdclark@gmail.com>
CC: David Airlie <airlied@linux.ie>
CC: Julia Lawall <julia.lawall@lip6.fr>
CC: linux-arm-msm@vger.kernel.org
CC: dri-devel@lists.freedesktop.org
CC: freedreno@lists.freedesktop.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Sean Paul <seanpaul@chromium.org>