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>
(cherry picked from commit 8197bb1bffbee6a7678d20b4c1e77940cdaa8640)
Signed-off-by: Alex Shi <alex.shi@linaro.org>