s390: cio: Delay uevents for subchannels

We often have the situation that we register a subchannel and start device
recognition, only to find out that the device is not usable after all, which
triggers an unregister of the subchannel.  This often happens on hundreds of
subchannels on a LPAR, leading to a storm of events which aren't of any use. 
Therefore, use uevent_suppress to delay the KOBJ_ADD uevent for a subchannel
until we know that its ccw_device is to be registered.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Eric Rannaud <eric.rannaud@gmail.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>