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>

drivers/isdn/hardware/mISDN/netjet.c

@@ -956,7 +956,7 @@ nj_release(struct tiger_hw *card)
 	}
 	if (card->irq > 0)
 		free_irq(card->irq, card);
-	if (card->isac.dch.dev.dev.class)
+	if (device_is_registered(&card->isac.dch.dev.dev))
 		mISDN_unregister_device(&card->isac.dch.dev);
 
 	for (i = 0; i < 2; i++) {