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>