mtd: atmel_nand: add a definition for the oob reserved bytes

It's better to use a macro instead of just a number.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
diff --git a/drivers/mtd/nand/atmel_nand.c b/drivers/mtd/nand/atmel_nand.c
index feaac90..0361c19 100644
--- a/drivers/mtd/nand/atmel_nand.c
+++ b/drivers/mtd/nand/atmel_nand.c
@@ -485,7 +485,7 @@
 	for (i = 0; i < ecc_len; i++)
 		layout->eccpos[i] = oobsize - ecc_len + i;
 
-	layout->oobfree[0].offset = 2;
+	layout->oobfree[0].offset = PMECC_OOB_RESERVED_BYTES;
 	layout->oobfree[0].length =
 		oobsize - ecc_len - layout->oobfree[0].offset;
 }
@@ -1254,7 +1254,8 @@
 		nand_chip->ecc.steps = mtd->writesize / sector_size;
 		nand_chip->ecc.total = nand_chip->ecc.bytes *
 			nand_chip->ecc.steps;
-		if (nand_chip->ecc.total > mtd->oobsize - 2) {
+		if (nand_chip->ecc.total >
+				mtd->oobsize - PMECC_OOB_RESERVED_BYTES) {
 			dev_err(host->dev, "No room for ECC bytes\n");
 			err_no = -EINVAL;
 			goto err;