s390/dasd: fix panic for failed online processing

Fix a panic that occurs for a device that got an error in
dasd_eckd_check_characteristics() during online processing.
For example the read configuration data command may have failed.

If this error occurs the device is not being set online and the earlier
invoked steps during online processing are rolled back. Therefore
dasd_eckd_uncheck_device() is called which needs a valid private
structure. But this pointer is not valid if
dasd_eckd_check_characteristics() has failed.

Check for a valid device->private pointer to prevent a panic.

Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>