OPP: Improve error handling in dev_pm_opp_of_cpumask_add_table()

The error handling wasn't appropriate in
dev_pm_opp_of_cpumask_add_table(). For example it returns 0 on success
and also for the case where cpumask is empty or cpu_device wasn't found
for any of the CPUs.

It should really return error on such cases, so that the callers can be
aware of the outcome.

Fix it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>