sclp: Fix uninitialized var in handle_write_event_buf().

Some gcc versions rightly complain about a possibly unitialized rc,
so let's move setting it before the QTAILQ_FOREACH().

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
