drm/msm: Fix iommu_map_sgtable() return value check and avoid WARN

drm/msm: Fix iommu_map_sgtable() return value check and avoid WARN

Commit "iommu: return full error code from iommu_map_sg[_atomic]()"
changed iommu_map_sgtable() to return an ssize_t and negative values
in error cases, rather than a size_t and a zero.

Store the return value in the appropriate type and in case of error,
return it rather than WARNing.

Fixes: ad8f36e4b6b1 ("iommu: return full error code from iommu_map_sg[_atomic]()")
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Patchwork: https://patchwork.freedesktop.org/patch/719685/
Message-ID: <20260421-iommu_map_sgtable-return-v1-3-fb484c07d2a1@nvidia.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>