s390x/css: clear IO irqs when generating IPI CRW

According to the Principles of Operation (more precisely the subsection
'Channel-Report Word'), a subchannel put into the installed parameters
initialized state is in the same state as after an I/O system reset (just
parameters possibly changed). This implies that any I/O interrupts for that
subchannel are no longer pending (as I/O system resets clear I/O
interrupts). Therefore, we need an interface to clear pending I/O
interrupts. Make css_generate_sch_crws clear the pending IO interrupts for
the subchannel.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
