cpufreq-dt: fix handling regulator_get_voltage() result

The function can return negative values so it should be assigned
to signed type.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci.

Link: http://permalink.gmane.org/gmane.linux.kernel/2038576


Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>