powerpc: Fix oops on some machines due to incorrect pr_debug()

Recently, a patch left DEBUG enabled in the powerpc common PCI code,
resulting in an old bug in a pr_debug() statement to show up and cause
a NULL dereference on some machines.

This fixes the pr_debug() statement and reverts to DEBUG not being
force-enabled in that file.

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