[MTD] [NAND] make s3c2410 indicate an error for multi-bit read errors

If there were multiple bit errors in the data s3c2410_nand_correct_data()
was returning 0 (no error) instead of -1, so the upper layers (like JFFS2)
would not know the data is corrupt.

Signed-off-by: Matt Reimer <mreimer@vpop.net>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>