PM / OPP: reuse find_device_opp() instead of duplicating code

Reuse find_device_opp() in opp_set_availability() instead of duplicating code.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 29df0ee1b14ab5cdc83c225258f42600825f45b2)
Signed-off-by: Alex Shi <alex.shi@linaro.org>