scsi: hisi_sas: only zero slot memory when reused

Currently the slot memory is zeroed when it is freed and also when it is
reused, like in hisi_sas_task_prep(). Optimise by avoiding the redundant
zeroing in the free.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>