scsi: libsas: Use enum for response frame DATAPRES field

As defined in table 126 of the SAS spec 1.1, use an enum for the DATAPRES
field, which makes reading the code easier.

Also change sas_ssp_task_response() to use a switch statement, which is
more suitable (than if-else), as suggested by Christoph.

Link: https://lore.kernel.org/r/1645112566-115804-3-git-send-email-john.garry@huawei.com


Suggested-by: Xiang Chen <chenxiang66@hisilicon.com>
Tested-by: Yihang Li <liyihang6@hisilicon.com>
Tested-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>