s390/ism: ignore some errors during deregistration

Prior to dma unmap/free operations the ism driver tries to ensure
that the memory is no longer accessed by the HW. When errors
during deregistration of memory regions from the HW occur the ism
driver will not unmap/free this memory.

When we receive notification from the hypervisor that a PCI function
has been detached we can no longer access the device and would never
unmap/free these memory regions which led to complaints by the DMA
debug API.

Treat this kind of errors during the deregistration of memory regions
from the HW as success since it is already ensured that the memory
is no longer accessed by HW.

Reported-by: Karsten Graul <kgraul@linux.ibm.com>
Reported-by: Hans Wippel <hwippel@linux.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>