manager: make sure modems get exported when all their ports are finished

My LG Rumor2 phone (ttyACM0 - AT, ttyUSB0 - QCDM) was never getting
exported after the QCDM port finished probing.  Not entirely sure
why, but we should check whether we need to export a modem or not
any time a port gets grabbed.

src/mm-manager.c

@@ -384,9 +384,13 @@ grab_port (MMManager *manager,
              mm_base_modem_get_device (modem),
              g_udev_device_get_name (device));
 
+    if (existing) {
+        g_assert (existing == modem);
+        check_export_modem (manager, modem);
+    } else {
         /* If the modem was just created, store it */
-    if (!existing)
         add_modem (manager, modem, plugin);
+    }
 }
 
 static GUdevDevice *