s390/cio: invalidate cdev pointer before deregistration

Make sure that the cdev pointer for IO subchannels is set to NULL when
we deregister the device (and release its last reference). This will
fix a bug were another process operates on an already freed ccw device.

Acked-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>