Fix kernel panic when unload bh_threaded module

Ensuring the free_irq function properly releases IRQs for buttons,
call free_irq() with button's device handler instead of NULL.

examples/bh_threaded.c

@@ -156,7 +156,7 @@ static int __init bottomhalf_init(void)
 /* cleanup what has been setup so far */
 #ifdef NO_GPIO_REQUEST_ARRAY
 fail4:
-    free_irq(button_irqs[0], NULL);
+    free_irq(button_irqs[0], &buttons[0]);
 
 fail3:
     gpio_free(buttons[1].gpio);
@@ -168,7 +168,7 @@ fail1:
     gpio_free(leds[0].gpio);
 #else
 fail3:
-    free_irq(button_irqs[0], NULL);
+    free_irq(button_irqs[0], &buttons[0]);
 
 fail2:
     gpio_free_array(buttons, ARRAY_SIZE(leds));
@@ -185,8 +185,8 @@ static void __exit bottomhalf_exit(void)
     pr_info("%s\n", __func__);
 
     /* free irqs */
-    free_irq(button_irqs[0], NULL);
-    free_irq(button_irqs[1], NULL);
+    free_irq(button_irqs[0], &buttons[0]);
+    free_irq(button_irqs[1], &buttons[1]);
 
 /* turn all LEDs off */
 #ifdef NO_GPIO_REQUEST_ARRAY