[tests] Fixed "test_restoring_deleted_device" test

The test was failing because of changes in openwisp-controller.
openwisp/openwisp-controller#962

openwisp_firmware_upgrader/tests/test_selenium.py

@@ -95,7 +95,7 @@ def test_restoring_deleted_device(self):
             reverse(f'admin:{self.config_app_label}_device_delete', args=[device.id])
         )
         self.web_driver.find_element(
-            by=By.XPATH, value='//*[@id="content"]/form/div/input[2]'
+            by=By.CSS_SELECTOR, value='#content form input[type="submit"]'
         ).click()
         self.assertEqual(Device.objects.count(), 0)
         self.assertEqual(DeviceConnection.objects.count(), 0)