s390/cio: get rid of static console subchannel

Remove the static console subchannel (and friends) and use dynamic
allocation for these structures. With this change the console
subchanel is treated (mostly) like any other subchannel and we can
remove some special cases.

Reviewed-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>