scsi: hisi_sas: use safe BITS_PER_BYTE for slot tag size calculation

The memory calculation for the tags bitmap should use BITS_PER_BYTE
macro instead of coincidental same value of sizeof(unsigned long).

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>