net/af_iucv: always register net_device notifier

net/af_iucv: always register net_device notifier

Even when running as VM guest (ie pr_iucv != NULL), af_iucv can still
open HiperTransport-based connections. For robust operation these
connections require the af_iucv_netdev_notifier, so register it
unconditionally.

Also handle any error that register_netdevice_notifier() returns.

Fixes: 9fbd87d41392 ("af_iucv: handle netdev events")
Signed-off-by: Julian Wiedmann <[email protected]>
Reviewed-by: Ursula Braun <[email protected]>
Signed-off-by: David S. Miller <[email protected]>