wwan: rework setting modem's data-port
Depending on the bearer's configuration method, the data-port is either a networking interface, or an tty for ppp. Let's treat them strictily separate. Also, rework how NM_MODEM_DATA_PORT was used in both contexts. Instead, use the that we actually care about. Also, when nm_device_set_ip_ifindex() fails, fail activation right away. Also, we early try to resolve the network interface's name to an ifindex. If that fails, the device is already gone and we fail early.
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