serial: sh-sci: Use sg_dma_len() and sg_dma_address()

Convert the dmaengine code in sh-sci.c from
directly accessing the struct scatterlist members
length, dma_length and dma_address to using the
macros sg_dma_len() and sg_dma_address().

Makes the code build for ARM.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>