[CELL] cbe_cpufreq: fix minor issues

Minor issues have been fixed:
* added a missing call to of_node_put()
* signedness of a function parameter
* added some line breaks
* changed global pmi_frequency_limit to a
  per node pmi_slow_mode_limit array

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>