[MTD] Deal correctly with NOR chips which are smaller than the map window

We used to calculate the number of chips to be zero, allocate an array
of that size, then nasty things would happen when we attempt to access
the first object in that zero-sized array.

Now, if the number of _full_ chips that would fit into the map is zero,
we allocate an array of one anyway, and then artificially reduce the
total size of the resulting MTD device to fit in the map.

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