s390/irq: use irq 0

Irq 0 is currently unused on s390. Since there is no reason to
do this start counting at the beginning and gain an additional
irq. Also correctly report the smallest usable irq number for
dynamic allocation.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>