powerpc/pseries: fix a potential memory leak

In case we have a full node name like /foo/bar and /foo is not found the
parent_path left unfreed. So, free a memory before return to a caller.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>