iommu: dma-mapping-fast: Fix error code for dma_map_sg

Zero indicates an error. Values less than zero trigger a bug, which is
undesirable.

Change-Id: I36296eeaf32a463503f7fe0a476ad2a35211c8e5
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
1 file changed