powerpc/numa: Avoid possible reference beyond prop. length in find_min_common_depth()

find_min_common_depth() was checking the property length incorrectly.
The value is in bytes not cells, and it is using the second entry.

Signed-off-By: Milton Miller <miltonm@bga.com>

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>