dmatest: fix automatic buffer unmap type

The dmatest code relies on the DMAEngine API to automatically call
dma_unmap_single() on src buffers. The flags it passes are incorrect,
fix them.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>