core: initialize l3cd dns-priority for ppp and wwan
For devices that configure IP by themselves (by returning "->ready_for_ip_config() = TRUE" and implementing ->act_stage3_ip_config()), we skip manual configuration. Currently, manual configuration is the only one that sets flag HAS_DNS_PRIORITY into the resulting l3cd.
So, the merged l3cd for such devices misses a dns-priority and is ignored by the DNS manager.
Explicitly initialize the priority to 0; in this way, the default value for the device will be set in the final l3cd during the merge.
Fixes: 58287cbc ('core: rework IP configuration in NetworkManager using layer 3 configuration')