mtd: OneNAND: fix incorrect bufferram offset

Fixes the case where CONFIG_MTD_ONENAND_2X_PROGRAM is set and
the real page size differs from mtd_info.writesize.

Signed-off-by: Mika Korhonen <mika.j.korhonen@gmail.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>