appletalk: Set error code if register_snap_client failed

If register_snap_client fails in atalk_init,
error code should be set, otherwise it will
triggers NULL pointer dereference while unloading
module.

Fixes: 9804501f ("appletalk: Fix potential NULL pointer dereference in unregister_snap_client")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>