s390/irq: rework irq subclass handling

Let's not add a function for every external interrupt subclass for
which we need reference counting. Just have two register/unregister
functions which have a subclass parameter:

void irq_subclass_register(enum irq_subclass subclass);
void irq_subclass_unregister(enum irq_subclass subclass);

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

drivers/s390/kvm/kvm_virtio.c

@@ -472,7 +472,7 @@ static int __init kvm_devices_init(void)
 
 	INIT_WORK(&hotplug_work, hotplug_devices);
 
-	service_subclass_irq_register();
+	irq_subclass_register(IRQ_SUBCLASS_SERVICE_SIGNAL);
 	register_external_interrupt(0x2603, kvm_extint_handler);
 
 	scan_devices();