[MTD] NAND: nandsim: support subpage write

As flash cannot do 0->1 bit transitions when programming, do not do this in
the simulator too. This makes nandsim able to accept subpage writes.

Signed-off-by: Artem Bityutskiy <dedekind@infradead.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>