[S390] cio: Use dev_{g,s}et_drvdata().

Also define helpers sch_{g,s}et_cdev() to make the intention more
clear.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>