iface-modem: allow reporting the previous power state after an update

Let the caller of mm_iface_modem_set_power_state() know which was the
power state in the device before the actual update happened.

This is useful in cases where we want to temporarily switch to a given
power state (e.g. low) before returning to the original power state.

src/mm-base-manager.c

@@ -908,7 +908,7 @@ shutdown_low_power_ready (MMIfaceModem   *modem,
 {
     g_autoptr(GError) error = NULL;
 
-    if (!mm_iface_modem_set_power_state_finish (modem, res, &error))
+    if (!mm_iface_modem_set_power_state_finish (modem, res, NULL, &error))
         mm_obj_info (ctx->self, "changing to low power state failed: %s", error->message);
 
     if (ctx->remove)