[S390] cio: introduce isc_(un)register functions.

This interface makes it easy for drivers to register usage of different
I/O interruption subclasses without needing to worry about possible
other users of the same isc.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>