isdn: mISDN: netjet: fix wrong check of device registration

The class is set in mISDN_register_device(), but if device_add() returns
error, it will lead to delete a device without added, fix this by using
device_is_registered() to check if the device is registered.

Fixes: a900845e ("mISDN: Add support for Traverse Technologies NETJet PCI cards")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>