mISDN: fix possible memory leak in mISDN_register_device()

In the Linux kernel, the following vulnerability has been resolved: mISDN: fix possible memory leak in mISDN_register_device() Afer commit 1fa5ae857bb1 ("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().