network: stop already running engines before updating MAC address

In NetworkdBridgeTests.test_bridge_configure_without_carrier of
systemd-networkd-tests.py

```
bridge99: MAC address: 2e:3a:ec:4d:d3:62
Assertion 'sd_ipv4ll_is_running(ll) == 0' failed at src/libsystemd-network/sd-ipv4ll.c:110, function int sd_ipv4ll_set_mac(sd_ipv4ll *, const struct ether_addr *)(). Ignoring.
bridge99: Could not update MAC address in IPv4LL client: Device or resource busy
```