device: fix check on device state in carrier_changed()

device: fix check on device state in carrier_changed()

The function can now be called on unrealized devices before the
initial unmanaged flags are set and for those devices
nm_device_get_managed() will return TRUE. Since we only accept
states > UNMANAGED, return early when the condition is not met.

Fixes the following failed assertion:

carrier_changed: assertion 'priv->state >= NM_DEVICE_STATE_UNAVAILABLE' failed

https://bugzilla.gnome.org/show_bug.cgi?id=760844