Draft: iface-modem: Increase unlock_required retries

If a SIM is known to exist, for e.g. if it was created during load_sim_slots, persist a few more times before giving up on the SIM to be ready. There are modems on which the SIM takes more than 15s to be ready, luckily, they happen to be QMI modems where the SIM's iccid in load_sim_slots lets us know that there is a SIM.