powerpc: Use sg->dma_length in sg_dma_len() macro on 32-bit

Currently, the 32-bit code uses sg->length instead of sg->dma_lentgh
to report sg_dma_len.  However, since the default dma code for 32-bit
(the dma_direct case) sets dma_length and length to the same thing,
we should be able to use dma_length there as well.  This gets rid of
some 32-vs-64-bit ifdefs, and is needed by the swiotlb code which
actually distinguishes between dma_length and length.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>