mtd: atmel_nand: fix support for CPUs that do not support DMA access

use_dma was always "1" even if the CPU does not support DMA

Tested on AT91SAM9261EK by Jean-Christophe PLAGNIOL-VILLARD

Reported-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Hong Xu <hong.xu@atmel.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
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