[MTD NAND] Reduce paranoia level when scanning for bad blocks on virgin chips

We were scanning for 0xFF through the entire chip -- which takes a while
when it's a 512MiB device as I have on my current toy. The specs only say
we need to check certain bytes -- so do only that.

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