net: ipconfig: Wait for deferred device probes

If network device drives are using deferred probing, it was possible
that waiting for devices to show up in ipconfig was already over,
when the device eventually showed up. By calling wait_for_device_probe()
we now make sure deferred probing is done before checking for available
devices.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>