The Bluetooth DUN device's NMModem would signal the reset of ifindex to zero
when it's disconnected and the NMDeviceBt would accordingly update the
bluetooth device's ip ifindex. This is not okay since commit ab4578302d
('device: refactor nm_device_set_ip_ifindex() and set_ip_iface()') which,
although claiming to be a refactoring, made such use of
nm_device_set_ip_ifindex() illegal. Resetting the ifindex is anyway not
necessary, since it's taken care of _cleanup_generic_post().
Let's leave the ifindex alone once the device is activated, in a manner
analogous to what NMDeviceModem.
Fixes: ab4578302d ('device: refactor nm_device_set_ip_ifindex() and set_ip_iface()')
Fixes: 78ca2a70c7 ('device: don't set invalid ip-iface'):
(cherry picked from commit a5ca504b5b)
fb874e6cda