[S390] Improve AP bus device removal.

Added a call to device_unregister() in ap_scan_bus() to actively
remove unavailable AP bus devices with every bus scan. Previously
devices were only removed in ap_queue_message() or __ap_poll_all().

Signed-off-by: Ralph Wuerthner <rwuerthn@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>