PM / devfreq: exynos-ppmu: Fix refcount leak in of_get_devfreq_events

of_get_child_by_name() returns a node pointer with refcount
incremented, we should use of_node_put() on it when done.
This function only calls of_node_put() in normal path,
missing it in error paths.
Add missing of_node_put() to avoid refcount leak.

Fixes: f262f28c ("PM / devfreq: event: Add devfreq_event class")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>