dma: mv_xor: simplify dma_sync_single_for_cpu() calls

In mv_xor_memcpy_self_test() and mv_xor_xor_self_test(), all DMA
functions are called by passing dma_chan->device->dev as the 'device
*', except the calls to dma_sync_single_for_cpu() which uselessly goes
through mv_chan->device->pdev->dev.

Simplify this by using dma_chan->device->dev direclty in
dma_sync_single_for_cpu() calls.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>