scsi: target/tcmu: Fix queue_cmd_ring() declaration

This patch does not change any functionality but avoids that sparse
complains about the queue_cmd_ring() function and its callers.

Fixes: 6fd0ce79 ("tcmu: prep queue_cmd_ring to be used by unmap wq")
Reviewed-by: David Disseldorp <ddiss@suse.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>