mtd: subpagetest: fix wrong written check in function write_eraseblock2

Write size in function write_eraseblock2 is subpgsize * k.
It is wrong to check whether written is equal to subpgsize after each
mtd_write.

Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>