ion: convert sg_dma_len(sg) to sg->length

ion is always dealing with the allocation and not the mapping,
so it should always be using sg->length and not sg->dma_length.

Change-Id: Id9b07f1196b2bafe04636fa1aa46dfc84d003cf0
Signed-off-by: Colin Cross <ccross@android.com>
Git-commit: c1d3cea1d0b985a78a87675a07c2dab865a416f1
Git-repo: http://android.googlesource.com/kernel/common/


Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>