pc-bios/s390-ccw: Device detection in higher subchannel sets

pc-bios/s390-ccw: Device detection in higher subchannel sets

If no bootdevice was specified, we try to autodetect a suitable IPL
device. Current code only searched in subchannel set 0; extend this
search to higher subchannel sets as well.

Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>