scsi: core: Refactor scsi_mq_setup_tags function

shost->tag_set is used too many times, introduce temporary parameter
tag_set instead of &shost->tag_set.

Link: https://lore.kernel.org/r/20200518074732.39679-1-yebin10@huawei.com
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>