s390/pci: fix handling of PEC 306

In contrast to other hotplug events PEC 0x306 isn't about a single
but multiple devices. Also there's no information on what happened
to these devices. We correctly handled hotplug that way but failed
to handle hot-unplug. This patch addresses that and implements
hot-unplug of multiple devices via PEC 306.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>