ata: libata-scsi: Cleanup ata_scsi_start_stop_xlat()

Now that libata does its own internal device power mode management
through libata EH, the scsi disk driver will not issue START STOP UNIT
commands anymore. We can receive this command only from user passthrough
operations. So there is no need to consider the system state and ATA
port flags for suspend to translate the command.

Since setting up the taskfile for the verify and standby
immediate commands is the same as done in ata_dev_power_set_active()
and ata_dev_power_set_standby(), factor out this code into the helper
function ata_dev_power_init_tf() to simplify ata_scsi_start_stop_xlat()
as well as ata_dev_power_set_active() and ata_dev_power_set_standby().

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Tested-by: Chia-Lin Kao (AceLan) <acelan.kao@canonical.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>