exynos-cpufreq: fix false return check from "regulator_set_voltage"

Currently, code checks false return value from "regulator_set_voltage"
to show failure message. Modify the code to check proper return
value from "regulator_set_voltage".

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