contrib/gsm_sim: extend PDP support

Improve it just a bit to make ModemManager 1.18 happy.
5 jobs for !846 with lr/gsm-sim-pdp in 6 minutes and 26 seconds
latest detached
Status Name Job ID Coverage
  Test
passed TestResults #14324600
manual

00:05:21

passed UnitTests #14324599

00:01:05

 
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/1920/

failed https://jenkins-networkmanager.apps.ocp.ci.centos.org/job/NetworkManager-test-mr/735/

04:41:22

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/1920/

02:01:58