s390/cio: derive cdev information only for IO-subchannels

cdev->online for the purge function must not be checked for the
non-IO subchannel type. Make sure that we are deriving the cdev only
from sch-type SUBCHANNEL_TYPE_IO.

Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>