[S390] cio: Fix handling of interrupt for csch().

Wipe internal irb if the clear function bit is set before accumulating
bits from the irb in order to follow hardware behaviour.

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