scsi: hisi_sas: Modify the file permissions of trigger_dump to write only

The trigger_dump file is only used to manually trigger the dump, and did
not provide a read callback function for it, so its file permission
setting to 600 is wrong,and should be changed to 200.

Link: https://lore.kernel.org/r/1579522957-4393-5-git-send-email-john.garry@huawei.com
Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>