dmaengine: imx-sdma: Save imx_dma_data into sdmac

The filter() function is currently called by xlate() while it transfers
imx_dma_data as a local variable to the filter() but releases the data
right after returning a DMA channel pointer, which results chan->private
pointing an invalid memory space.

So this patch just stores the imx_dma_data into sdmac to make usre the
private pointer valid as long as the channel exists.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>