ion: do not use sg_dma_len() macro

Use of sg_dma_len() is not portable to 64 bit and is not used any more
by ion or iommu. Replace the usages by direct access of the length
structure member.

Change-Id: I6c18b2a77e288be312448c36f8ed3ad6db90ac0f
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>