mtd: atmel_nand: modify test case for using DMA operations

We have better performances not using DMA for oob operations.
Modify size test so that it is using DMA for size greater than oobsize.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>