PM / devfreq: Add missing error code in devfreq_add_device()

Set err code in the error path before jumping to the end of the function.

Fixes: 4dc3bab8

 ("PM / devfreq: Add support delayed timer for polling mode")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>