s390/cio: Fix missing subchannels after CHPID configure on

Performing a Channel-Path configure on operation on a Channel-Path ID
(CHPID) does not trigger a scan for subchannels that might have become
available through that CHPID. As a result, some subchannels and
associated I/O devices might be missing. Fix this by adding the missing
scan.

This problem was introduced by commit c820de39, "[S390] cio: Rework
css driver.", but wasn't noticed earlier because the machine usually
also generates a Channel-Report-Word when the first CHPID of a
subchannel is configured on, resulting in a separate scan for that
subchannel. The problem only becomes apparent when this first CHPID is
not working properly and additional working CHPIDs are subsequently
configured on without any effect on the availability of the affected
subchannel.

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