devfreq: rk3399_dmc: Remove dangling rcu_read_unlock()

This call never had the rcu_read_lock() counterpart. Remove the unlock
part as well.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>