hisi_acc_vfio_pci: bugfix cache write-back issue

hisi_acc_vfio_pci: bugfix cache write-back issue

At present, cache write-back is placed in the device data
copy stage after stopping the device operation.
Writing back to the cache at this stage will cause the data
obtained by the cache to be written back to be empty.

In order to ensure that the cache data is written back
successfully, the data needs to be written back into the
stop device stage.

Fixes: b0eed085903e ("hisi_acc_vfio_pci: Add support for VFIO live migration")
Signed-off-by: Longfang Liu <liulongfang@huawei.com>
Reviewed-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Link: https://lore.kernel.org/r/20250510081155.55840-4-liulongfang@huawei.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>