iface-modem-simple: don't assert ongoing connect cancellable

[11642]: <debug> [1573665255.321490] Couldn't reload current power state: QMI operation failed: Transaction timed out
    [11642]: <debug> [1573665255.321697] No need to change power state: already in 'on' power state
    [11642]: <debug> [1573665255.330864] Modem (Quectel) '/sys/devices/platform/ehci-platform/usb1/1-1' completely disposed
    **
    ERROR:mm-iface-modem-simple.c:44:private_free: assertion failed: (!priv->ongoing_connect)

If the modem goes away in the middle of a connection attempt, there's
no explicit connection cancellation performed, we just cleanup the
modem object. In this case, the ongoing attempt cancellable will still
exist, so just clean it up as well.

src/mm-iface-modem-simple.c

@@ -41,7 +41,7 @@ typedef struct {
 static void
 private_free (Private *priv)
 {
-    g_assert (!priv->ongoing_connect);
+    g_clear_object (&priv->ongoing_connect);
     g_slice_free (Private, priv);
 }