mtd: onenand: bugfix for 2x mode bad block handling

This bug becomes visible in 2x mode when chip->writesize
is different from mtd->writesize (= 2 * chip->writesize).
At this case the bad block information is read from
the first and the third physical pages instead of
the first and the second as specification states.

Signed-off-by: Roman Tereshonkov <roman.tereshonkov@nokia.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>