mISDN: fix possible memory leak in mISDN_register_device()

Afer commit 1fa5ae85 ("driver core: get rid of struct device's
bus_id string array"), the name of device is allocated dynamically,
add put_device() to give up the reference, so that the name can be
freed in kobject_cleanup() when the refcount is 0.

Set device class before put_device() to avoid null release() function
WARN message in device_release().

Fixes: 1fa5ae85 ("driver core: get rid of struct device's bus_id string array")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>