s390/pci: remove hotplug slot when releasing the device

Centralize the removal so all paths are covered and the hotplug slot
will remain active until the device is really destroyed.

Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>