mtd: nftl: write support is broken

Write support is broken in NFTL. Fix it.

Signed-off-by:  <dimitri.gorokhovik@free.fr>
Cc: Tim Gardner <tim.gardner@canonical.com>
Cc: Scott James Remnant <scott@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>