modemmanager: reset to 'no' plmn if nothing is configured

If a configured 'plmn' is deleted from the configuration between a connection
setup, it will continue to be used because the modem remembers it.

Therefore, the 'plmn' stored in the modem must be deleted when a new
connection is established if it is no longer in the configuration.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
(cherry picked from commit fa6198e36b)

net/modemmanager/files/lib/netifd/proto/modemmanager.sh

@@ -673,11 +673,14 @@ proto_modemmanager_setup() {
 		[ "$?" -ne "0" ] && return 1
 	fi
 
-	[ -z "${plmn}" ] || {
+	if [ -z "${plmn}" ]; then
+		modemmanager_set_plmn "$device" "$interface" "" "$force_connection"
+		[ "$?" -ne "0" ] && return 1
+	else
 		echo "starting network registration with plmn '${plmn}'..."
 		modemmanager_set_plmn "$device" "$interface" "$plmn" "$force_connection"
 		[ "$?" -ne "0" ] && return 1
-	}
+	fi
 
 	# setup connect args; APN mandatory (even if it may be empty)
 	echo "starting connection with apn '${apn}'..."