powerpc/cacheinfo: Improve diagnostics about malformed cache lists

If we have a bug which causes us to start with the wrong kind of OF
node when linking up the cache tree, it's helpful for debugging to
print information about what we found vs what we expected. So replace
uses of WARN_ON_ONCE with WARN_ONCE, which lets us include an
informative message instead of a contentless backtrace.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190627051537.7298-4-nathanl@linux.ibm.com