[S390] cio: Fix incomplete commit for uevent suppression.

Commit fa1a8c23

 intended to
introduce uevent suppression for subchannels, but half of it was
lost somewhere. Now, we end up with two uevents for every registered
subchannel :( So we should better add the missing part from
http://marc.info/?l=linux-kernel&m=117515953113974&w=2.

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