iommu: msm: Fix __msm_dma_map_sg sg_tmp sanity check

Fix the sanity check in __msm_dma_map_sg to ensure that sg_tmp is always
valid.

Change-Id: Idfb242afa42ce07727458d9319aaa374ab3cb3ec
Signed-off-by: Liam Mark <lmark@codeaurora.org>