dmaengine: ioat: don't use DMA_ERROR_CODE

DMA_ERROR_CODE is not a public API and will go away.  Instead properly
unwind based on the loop counter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Acked-By: Vinod Koul <vinod.koul@intel.com>