spi: No need to assign dummy value in spi_unregister_controller()

The device_for_each_child() doesn't require the returned value to be checked.
Thus, drop the dummy variable completely and have no warning anymore:

drivers/spi/spi.c: In function ‘spi_unregister_controller’:
drivers/spi/spi.c:2480:6: warning: variable ‘dummy’ set but not used [-Wunused-but-set-variable]
  int dummy;
      ^~~~~
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2559,7 +2559,6 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 {
 	struct spi_controller *found;
 	int id = ctlr->bus_num;
-	int dummy;
 
 	/* First make sure that this controller was ever added */
 	mutex_lock(&board_lock);
@@ -2573,7 +2572,7 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 	list_del(&ctlr->list);
 	mutex_unlock(&board_lock);
 
-	dummy = device_for_each_child(&ctlr->dev, NULL, __unregister);
+	device_for_each_child(&ctlr->dev, NULL, __unregister);
 	device_unregister(&ctlr->dev);
 	/* free bus id */
 	mutex_lock(&board_lock);