s390/char: fix cdev_add usage

Function cdev_add does set cdev->dev, so there is no point in setting
it prior to calling this function.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>