dmaengine: mv_xor: Use correct device for DMA API

Using dma_dev->dev for mappings before it's assigned with the correct
device is unlikely to work as expected, and with future dma-direct
changes, passing a NULL device may end up crashing entirely. I don't
know enough about this hardware or the mv_xor_prep_dma_interrupt()
operation to implement the appropriate error-handling logic that would
have revealed those dma_map_single() calls failing on arm64 for as long
as the driver has been enabled there, but moving the assignment earlier
will at least make the current code operate as intended.

Fixes: 22843545 ("dma: mv_xor: Add support for DMA_INTERRUPT")
Reported-by: John David Anglin <dave.anglin@bell.net>
Tested-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>