dmaengine: xgene-dma: move unmap to before callback

Completion callback should happen after dma_descriptor_unmap() has
happened. This allow the cache invalidate to happen and ensure that
the data accessed by the upper layer is in memory that was from DMA
rather than stale data. On some architecture this is done by the
hardware, however we should make the code consistent to not cause
confusion.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Cc: Rameshwar Prasad Sahu <rsahu@apm.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>