cpufreq: cpufreq-cpu0: NULL is a valid regulator, part 2

Since the patch "cpufreq: cpufreq-cpu0: NULL is a valid regulator", cpu_reg
contains an error value if the regulator is not set, instead of NULL.
Accordingly, fix the remaining check for non-NULL cpu_reg.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>