cpufreq: cpufreq-cpu0: NULL is a valid regulator

Since NULL could in theory be a valid regulator we ought to check for
IS_ERR() rather than for NULL. In practice this is unlikely to be an
issue but it's better for neatness.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>