[S390] cio: Make ccw_device_register() static.

ccw_device_register() is only called from io_subchannel_register()
and io_subchannel_probe() and will never be called for possible
non-io subchannels.

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