mtd: OneNAND: Fix page offset handling at 2KiB pagesize

When use the 2KiB pagesize, it should be set the correct page offset.

Signed-off-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>