cpufreq: ACPI: policy->driver_data can't be NULL in ->exit()

Its always set by ->init() and so it will always be there in ->exit().
There is no need to have a special check for just that.

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