ata: do not schedule hot plug if it is a sas host

We've got a kernel panic when using sata disk with sas controller:

[115946.152283] Unable to handle kernel NULL pointer dereference at virtual address 000007d8
[115946.223963] CPU: 0 PID: 22175 Comm: kworker/0:1 Tainted: G   W OEL  4.14.0 #1
[115946.232925] Workqueue: events ata_scsi_hotplug
[115946.237938] task: ffff8021ee50b180 task.stack: ffff00000d5d0000
[115946.244717] PC is at sas_find_dev_by_rphy+0x44/0x114
[115946.250224] LR is at sas_find_dev_by_rphy+0x3c/0x114
......
[115946.355701] Process kworker/0:1 (pid: 22175, stack limit = 0xffff00000d5d0000)
[115946.363369] Call trace:
[115946.456356] [<ffff000008878a9c>] sas_find_dev_by_rphy+0x44/0x114
[115946.462908] [<ffff000008878b8c>] sas_target_alloc+0x20/0x5c
[115946.469408] [<ffff00000885a31c>] scsi_alloc_target+0x250/0x308
[115946.475781] [<ffff00000885ba30>] __scsi_add_device+0xb0/0x154
[115946.481991] [<ffff0000088b520c>] ata_scsi_scan_host+0x180/0x218
[115946.488367] [<ffff0000088b53d8>] ata_scsi_hotplug+0xb0/0xcc
[115946.494801] [<ffff0000080ebd70>] process_one_work+0x144/0x390
[115946.501115] [<ffff0000080ec100>] worker_thread+0x144/0x418
[115946.507093] [<ffff0000080f2c98>] kthread+0x10c/0x138
[115946.512792] [<ffff0000080855dc>] ret_from_fork+0x10/0x18

We found that Ding Xiang has reported a similar bug before:
https://patchwork.kernel.org/patch/9179817/



And this bug still exists in mainline. Since libsas handles hotplug and
device adding/removing itself, do not need to schedule ata hot plug task
here if it is a sas host.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Cc: Ding Xiang <dingxiang@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>