[S390] cio: fix orb initialization in cio_start_key

The functions cio_tm_start_key and cio_start_key use the same private
orb structure of a subchannel, so the orb needs to be cleared of old
data before it is used again. A respective memset is missing from
cio_start_key and hereby added.

Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>