powerpc/powernv/pci: fix iterator signedness

Unsigned type is always non-negative, so the loop could not end in case
condition is never true.

The problem has been detected using semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>