dmaengine: ste_dma40: Cleanup scatterlist layering violations

This dma engine driver directly accesses page_link assuming knowledge
that should be contained only in scatterlist.h.

We replace this access with a call to sg_chain which is equivalent.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Stephen Bates <sbates@raithlin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Per Förlin <per.forlin@axis.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>