HSI: core: Fix return freed object in hsi_new_client

cl is freed on error of calling device_register, but this
object is return later, which will cause uaf issue. Fix it
by return NULL on error.

Signed-off-by: Chengfeng Ye <cyeaa@connect.ust.hk>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>