[MTD] [NOR] Fix overflow check in jedec_probe

Having laid the code out so that it's easier to read instead of sticking
to the 80-column guideline even when it doesn't make sense, a bug is
immediately spotted... we were only checking _one_ of the unlock
addresses to see if it runs off the end of the map.

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