scsi: hisi_sas: Drain bcast events in hisi_sas_rescan_topology()

In resetting the controller, SATA devices may be lost.

The issue is that when we insert the bcast events to rescan the topology in
hisi_sas_rescan_topology(), when we subsequently nexus reset the SATA
devices in hisi_sas_async_I_T_nexus_reset(), there is a small timing window
in which the remote phy is down and we process the bcast event (meaning
that libsas judges that the disk is lost).

Ensure that all bcast events are processed prior to the nexus reset to
close this window.

Link: https://lore.kernel.org/r/1662378529-101489-4-git-send-email-john.garry@huawei.com


Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>