sony-laptop: use platform_device_unregister in sony_pf_remove

platform_device_unregister calls platform_device_del and platform_device_put,
thus this change is logically equivalent to original code.

I made this change because the documents in platform.c shows that:
platform_device_del and platform_device_put must _only_ be externally called
in error cases.  All other usage is a bug.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

drivers/platform/x86/sony-laptop.c

@@ -561,8 +561,7 @@ static void sony_pf_remove(void)
 	if (!atomic_dec_and_test(&sony_pf_users))
 		return;
 
-	platform_device_del(sony_pf_device);
-	platform_device_put(sony_pf_device);
+	platform_device_unregister(sony_pf_device);
 	platform_driver_unregister(&sony_pf_driver);
 }