Fix nmstate: use primary interface for custom route

The dummy values that we use disable ipv4 on bond0.200 on purpose, but the static route is configured to use that interface resulting in error

NmstateError: InvalidArgument: The next hop interface of desired Route 'destination: 2.2.2.0/30 next-hop-interface: bond0.200 next-hop-address: 192.168.10.1 metric: 150 table-id: 254' has been marked as IPv4 disabled