mtd: nand: s3c2410: fix bug in s3c2410_nand_correct_data()

If there is only one bit difference in the ECC, the function should
return 1.
The result of "diff0 & ~(1<<fls(diff0))" is equal to diff0, so the
function actually returns -1.

Signed-off-by: Zhaoxiu Zeng <zhaoxiu.zeng@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>