MBIM stop script: remove IP address

The CI detects whether a UE connected (and has a PDU session) by trying
to read the IP address. It therefore assumes that as long as the UE is
not connected, no IP would be present.

With the existing stop_quectel_mbim.sh script, this was not the case; it
would simply leave the IP address.  Modify this script to flush all IP
addresses on disconnect to remedy this.

ci-scripts/mbim_scripts/stop_quectel_mbim.sh

@@ -1,3 +1,7 @@
 set -x
 
 sudo mbimcli -p -d /dev/cdc-wdm0 --set-radio-state=off
+
+IF=wwan0
+sudo ip link set ${IF} down
+sudo ip addr flush dev ${IF}