async_memcpy: convert to dmaengine_unmap_data

Use the generic unmap object to unmap dma buffers.

Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Tomasz Figa <t.figa@samsung.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Reported-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
[bzolnier: add missing unmap->len initialization]
[bzolnier: fix whitespace damage]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
[djbw: add DMA_ENGINE=n support]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>