cpufreq: cpufreq-dt: Handle regulator_get_voltage() failure

In error cases regulator_get_voltage() returns a negative value.
So take care of it.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>