cpufreq: imx6q: add return value check for voltage scale

Add return value check for voltage scale when ARM clock
rate change fail.

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>