cpufreq: acpi-cpufreq: Fix up the handling of cpb sysfs attribute

The cpb sysfs attribute is only exposed by the ACPI cpufreq driver
after a runtime check.  For this purpose, the driver keeps a NULL
placeholder in its table of sysfs attributes and replaces the NULL
with a pointer to an attribute structure if it decides to expose
cpb.

That is confusing, so make the driver set the pointer to the cpb
attribute structure upfront and replace it with NULL if the
attribute should not be exposed instead.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>