mtd: onenand: omap2: Pass correct flags for prep_dma_memcpy

The commit converting the driver to DMAengine was missing the flags for
the memcpy prepare call.
It went unnoticed since the omap-dma driver was ignoring them.

Fixes: 3ed6a4d1

 (" mtd: onenand: omap2: Convert to use dmaengine for memcp")
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>