microblaze: Fix sg_dma_len() regression

The commit "asm-generic: add NEED_SG_DMA_LENGTH to define sg_dma_len()"
18e98307de0d746cb0845ebf66535ce2184c25a2 broke microblaze compilation.

dma_direct_map_sg() sets sg->dma_length, however microblaze doesn't
set NEED_SG_DMA_LENGTH so scatterlist strcutres doesn't include
dma_length.

sg->dma_length is always equal to sg->length on microblaze. So we
don't need to set set dma_length, that is, microblaze can simply use
sg->length.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Michal Simek <monstr@monstr.eu>