cpufreq: exit() callback is optional

The exit() callback is optional and shouldn't be called without checking
a valid pointer first.

Also, we must clear freq_table pointer even if the exit() callback isn't
present.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Fixes: 91a12e91 ("cpufreq: Allow light-weight tear down and bring up of CPUs")
Fixes: f339f354 ("cpufreq: Rearrange locking in cpufreq_remove_dev()")
Reported-by: Lizhe <sensor1010@163.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>