mtd: nand: sunxi: fix subpage write

Implement ecc->write_subpage() to prevent core code from assigning this
hook to nand_write_subpage_hwecc(). This default implementation tries
to call ecc->hwctl() which in our case is NULL, thus leading to a NULL
pointer dereference.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>