scsi: NCR5380: Use default @max_active for hostdata->work_q

hostdata->work_q only hosts a single work item, hostdata->main_task, and
thus doesn't need explicit concurrency limit. Let's use the default
@max_active. This doesn't cost anything and clearly expresses that
@max_active doesn't matter.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Finn Thain <fthain@linux-m68k.org>
Acked-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: linux-scsi@vger.kernel.org
Cc: linux-kernel@vger.kernel.org