mtd: onenand: omap2: Disable DMA for HIGHMEM buffers

dma_map_single does not work for vmalloc-ed buffers,
so disable DMA in this case.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reported-by: "H. Nikolaus Schaller" <hns@goldelico.com>
Tested-by: "H. Nikolaus Schaller" <hns@goldelico.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>