[S390] cio: move irritating comment.

Due to former patches a comment and device id initialization were
split from the addressed function call in io_subchannel_probe.

Move it back to where it belongs.

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