cpufreq: governor: CPUFREQ_GOV_LIMITS never fails

None of the cpufreq governors currently in the tree will ever fail
an invocation of the ->governor() callback with the event argument
equal to CPUFREQ_GOV_LIMITS (unless invoked with incorrect arguments
which doesn't matter anyway) and had it ever failed, the result of
it wouldn't have been very clean.

For this reason, rearrange the code in the core to ignore the return
value of cpufreq_governor() when called with event equal to
CPUFREQ_GOV_LIMITS.

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