[MTD] OneNAND: Subpage write returned incorrect length written

When a write is done, the length written is returned.  When a
single subpage is written the length returned should be the
subpage size, however the page size was being returned.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>