s390: Move css limits from drivers/s390/cio/ to include/asm/.

There's no need to keep __MAX_SUBCHANNEL and __MAX_SSID private to the
common I/O layer when __MAX_CSSID is usable by everybody.

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