cpufreq: drop !cpufreq_driver check from cpufreq_parse_governor()

Driver is guaranteed to be present on a call to cpufreq_parse_governor()
and there is no need to check for !cpufreq_driver. Drop it.

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