atmel_nand: use the definition: PMECC_OOB_RESERVED_BYTES instead magic number

As atmel_nand_ecc.h is sync with v4.1 kernel, which adds the
PMECC_OOB_RESERVED_BYTES. So use it in the driver.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>