s390/pci: improve unreg_ioat error handling

DMA tables are freed in zpci_dma_exit_device regardless of the return
code of zpci_unregister_ioat. This could lead to a use after free. On
the other hand during function hot-unplug, zpci_unregister_ioat will
always fail since the function is already gone.

So let zpci_unregister_ioat report success when the function is gone
but don't cleanup the dma table when a function could still have it
in access.

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>