cpufreq: Use rwsem for protecting critical sections

Critical sections of the cpufreq core are protected with the help of
the driver module owner's refcount, which isn't the correct approach,
because it causes rmmod to return an error when some routine has
updated that refcount.

Let's use rwsem for this purpose instead.  Only
cpufreq_unregister_driver() will use write sem
and everybody else will use read sem.

[rjw: Subject & changelog]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>