PM / OPP replace kfree_rcu() with call_srcu() in opp_set_availability()

This existed before we introduced call_srcu() in opp layer to synchronize with
srcu_notifier_call_chain() while removing OPPs. And is a potential bug which
wasn't noticed earlier.

Let fix it as well by using the right API to free OPP.

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