mtd: nand: omap2: Fix high memory dma prefetch transfer

Based on DMA documentation and testing using high memory buffer when doing
dma transfers can lead to various issues including kernel panics.

To workaround this simply use cpu copy.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>