scsi: libsas: only clear phy->in_shutdown after shutdown event done

When the event queue is full of phy up and down events and reached the
threshold, we will queue a shutdown-event, and set phy->in_shutdown so that
we will not queue a shutdown-event again. But before the shutdown-event can
be executed, every phy-down event will clear
phy->in_shutdown and a new shutdown-event will be queued. The queue will
be full of these shutdown-events.

Fix this by only clear phy->in_shutdown in sas_phye_shutdown(), that is
after the first shutdown-event has been executed.

Fixes: f12486e0

 ("scsi: libsas: shut down the PHY if events reached the threshold")
Signed-off-by: Jason Yan <yanaijie@huawei.com>
CC: John Garry <john.garry@huawei.com>
CC: Johannes Thumshirn <jthumshirn@suse.de>
CC: Ewan Milne <emilne@redhat.com>
CC: Christoph Hellwig <hch@lst.de>
CC: Tomas Henzl <thenzl@redhat.com>
CC: Dan Williams <dan.j.williams@intel.com>
CC: Hannes Reinecke <hare@suse.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>