dell-laptop: Fix platform device unregistration

dell-laptop currently fails to clean up its platform device correctly.
Make sure that it's unregistered.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
diff --git a/drivers/platform/x86/dell-laptop.c b/drivers/platform/x86/dell-laptop.c
index 9d7e0be..b4c2190 100644
--- a/drivers/platform/x86/dell-laptop.c
+++ b/drivers/platform/x86/dell-laptop.c
@@ -474,6 +474,10 @@
 	i8042_remove_filter(dell_laptop_i8042_filter);
 	backlight_device_unregister(dell_backlight_device);
 	dell_cleanup_rfkill();
+	if (platform_device) {
+		platform_device_del(platform_device);
+		platform_driver_unregister(&platform_driver);
+	}
 }
 
 module_init(dell_init);