powerpc/pseries: handle nodes without '/'

In case we have node without '/' strrchr() returns NULL which might lead to
crash. Replace strrchr() by kbasename() and modify condition to avoid such
behaviour.

Suggested-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>