cpufreq: imx6q: remove unused code

In voltage scale down path, the return value is NOT
used at all, remove them to simplify the code.

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