cpufreq: Fix clamp_val() usage in cpufreq_driver_fast_switch()

The return value of clamp_val() has to be stored actually.

Fixes: b7898fda (cpufreq: Support for fast frequency switching)
Reported-by: Steve Muckle <steve.muckle@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>