modem: use set conn.interface upon completing the CDMA connections

There doesn't seem to be a better way to pinpoint a CDMA connection to a
device. This will have to do for now.
12 jobs for lr/complete-conn in 27 minutes and 10 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #286850
allowed to fail
checkpatch

00:01:05

passed #286857
t_debian:9

00:13:43

passed #286859
t_debian:sid

00:11:18

manual #286858
allowed to fail manual
t_debian:testing
passed #286851
t_fedora:28

00:19:51

passed #286852
t_fedora:29

00:27:10

passed #286853
t_fedora:30

00:21:17

manual #286854
allowed to fail manual
t_fedora:rawhide
passed #286855
t_ubuntu:16.04

00:13:47

passed #286856
t_ubuntu:18.04

00:11:37

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

05:58:13