cpufreq: allow cpufreq_generic_suspend() to work without suspend frequency

Some cpufreq drivers may set suspend frequency only for
selected setups but still would like to use the generic
suspend handler.  Thus don't treat !policy->suspend_freq
condition as an incorrect one.

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