[S390] cio: Register all subchannels.

Register all valid subchannels, not only I/O subchannels.
Move I/O subchannel specific initialization to io_subchannel_probe().

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