ide: Remove wrong assertion

The Bus Master IDE Active bit (BM_STATUS_DMAING) is not only set when
the request is still in flight, but also when it has completed and the
size of the physical memory regions in the PRDT was larger than the
transfer size.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>