[MTD] [OneNAND] Return only negative error codes

The OneNAND driver was confusing JFFS2 by returning positive error
codes.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>