scsi: hisi_sas: Initialise devices in .slave_alloc callback

Perform driver-specific SCSI device initialization in the designated SCSI
midlayer callback instead of relying on the libsas "device found" callback.

The SCSI midlayer .slave_alloc interface is called prior to sending any I/O
to the device.

Link: https://lore.kernel.org/r/1634041588-74824-2-git-send-email-john.garry@huawei.com
Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>