s390/irq: enforce correct irqclass_sub_desc array size

Add a BUILD_BUG_ON() to enforce that irqclass_sub_desc contains the required
number of defined interrupt descriptions and won't be filled up with zeros.

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