thermal: sysfs: Fix cooling_device_stats_setup() error code path

commit d5a8aa5d7d80d21ab6b266f1bed4194b61746199 upstream.

If cooling_device_stats_setup() fails to create the stats object, it
must clear the last slot in cooling_device_attr_groups that was
initially empty (so as to make it possible to add stats attributes to
the cooling device attribute groups).

Failing to do so may cause the stats attributes to be created by
mistake for a device that doesn't have a stats object, because the
slot in question might be populated previously during the registration
of another cooling device.

Fixes: 8ea22951 ("thermal: Add cooling device's statistics in sysfs")
Reported-by: Di Shen <di.shen@unisoc.com>
Tested-by: Di Shen <di.shen@unisoc.com>
Cc: 4.17+ <stable@vger.kernel.org> # 4.17+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>