spapr-vscsi: fix CRQ status

Normally VIOSRP_OK (0) means success and non-zero value means error
except VIOSRP_OK2 (0x99) which is another success code by weird accident.

This uses 0 as success code always as some guests do not cope with
the 0x99 value well. The existing linux driver checks for both VIOSRP_OK
and VIOSRP_OK2 since 2.6.32.

This returns non-zero code (VIOSRP_ADAPTER_FAIL == 0x10) on errors which
can only happen if DMA write failed.

Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
