sh: remove unused DMA device pointer from SIU platform data

The SIU ALSA driver is not using the DMA device pointer for DMA channel
filtering any more, it can be now removed.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>