mtd: return success on blktrans_ioctl

There was a break missing so we returned -ENOTTY on success instead of
zero.  This was introduced by 048d8719: "mtd: blktrans: Hotplug fixes"

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>