scsi: dc395x: fix dma API usage in srb_done

We can't just transfer ownership to the CPU and then unmap, as this will
break with swiotlb.

Instead unmap the command and sense buffer a little earlier in the I/O
completion handler and get rid of the pci_dma_sync_sg_for_cpu call
entirely.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>