s390/irq: use designated initializers for irq class array

Use designated initializers for the irq class array in irq.c so
it's always guaranteed that the order of elements is equal to
their corresponding parts in irq.h.

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