mtd: mtdconcat: fix NAND OOB write

Currently mtdconcat is broken for NAND. An attemtpt to create
JFFS2 filesystem on concatenation of several NAND devices fails
with OOB write errors. This patch fixes that problem.

Signed-off-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: stable@kernel.org
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>