mtd: nand: Fix a couple error codes

We accidentally return 1 on error instead of proper error codes.

Fixes: 07b23e3db9ed ("mtd: nand: Cleanup/rework the atmel_nand driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>