ntb_perf: Fix an error code in perf_copy_chunk()

We accidentally return success if dmaengine_submit() fails.  The fix is
to preserve the error code from dma_submit_error().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>