scsi: acornscsi: Remove scsi_cmd_to_tag() reference

Commit 756fb6a8 ("scsi: acornscsi: Remove tagged queuing vestiges")
mistakenly introduced a reference to function scsi_cmd_to_tag(). This
function does not exist as it was removed from an earlier series version
when I upstreamed the named commit - originally authored By Hannes - but
this reference still remained.

Fix by replacing the reference to scsi_cmd_to_tag() with
scsi_cmd_to_rq(scsi_scmd)->tag, which scsi_cmd_to_tag() was a wrapper for.

Link: https://lore.kernel.org/r/1633002717-79765-1-git-send-email-john.garry@huawei.com
Fixes: 756fb6a8

 ("scsi: acornscsi: Remove tagged queuing vestiges")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>