dmaengine: dmatest: Restore "memcpy" as default mode

Commit 0d4cb44da6ca0e8 ("dmaengine: dmatest: Add support for
scatter-gather DMA mode") changes default "dmatest" behavior by
changing default mode from "memcpy" to "scatter-gather".
Now "memcpy" gets back as default mode.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>