s390/ap: enable sysfs attribute scans to force AP bus rescan

This patch switches the sysfs attribute /sys/bus/ap/scans
from read-only to read-write. If there is something written
to this attribute, an AP bus rescan is forced. If an AP
bus scan is triggered this way a debug feature entry line
reports this in /sys/kernel/debug/s390dbf/ap/sprintf.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Jakob Naucke <naucke@linux.ibm.com>
Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>