PM / devfreq: Fix leak in devfreq_dev_release()

srcu_init_notifier_head() allocates resources that need to be released
with a srcu_cleanup_notifier_head() call.

Reported by kmemleak.

Fixes: 0fe3a664

 ("PM / devfreq: Add new DEVFREQ_TRANSITION_NOTIFIER notifier")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>