[S390] cio: provide helper functions for fcx enabled I/O

Provide functions which can be used to incrementally construct fcx
enabled I/O control blocks.

Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>