PM / OPP: Return suspend_opp only if it is enabled

There is no point returning suspend_opp, if it is disabled by the core.
As we can't use it at all. Fix it.

Fixes: 4eafbd15 ("PM / OPP: add dev_pm_opp_get_suspend_opp() helper")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>