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>
diff --git a/drivers/mtd/nand/atmel_nand.c b/drivers/mtd/nand/atmel_nand.c
index 852883b..73fd403 100644
--- a/drivers/mtd/nand/atmel_nand.c
+++ b/drivers/mtd/nand/atmel_nand.c
@@ -901,7 +901,7 @@
 			return -EINVAL;
 		}
 
-		if (nand->ecc.bytes > mtd->oobsize - 2) {
+		if (nand->ecc.bytes > mtd->oobsize - PMECC_OOB_RESERVED_BYTES) {
 			dev_err(host->dev, "No room for ECC bytes\n");
 			return -EINVAL;
 		}