s390/ap: rework ap_scan_bus() to return true on config change

The AP scan bus function now returns true if there have
been any config changes detected. This will become
important in a follow up patch which will exploit this
hint for further actions. This also required to have
the AP scan bus timer callback reworked as the function
signature has changed to bool ap_scan_bus(void).

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>