s390/pci: improve handling of hotplug event 0x301

Hypervisors may deliver event 0x301 not only for standby
but also for reserved devices.
Just handle event 0x301 regardless of the device's state.

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