spi: core: Pass correct device to dma_map_sg()

According to Documentation/dmaengine.txt, scatterlists must be mapped
using the DMA struct device.

However, "dma_chan.dev->device" is the sysfs class device's device.
Use "dma_chan.device->dev" instead, which is the real DMA device's device.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>
(cherry picked from commit 3fc25421)
(cherry picked from commit ff593afabbe51959293cf8434bbe72a3db24dda5)