ide-tape: make room for packet command ids in rq->cmd

ide-tape uses rq->cmd for special commands from the chrdev interface so move
those to byte 13 (BLK_MAX_CDB = 16) since a packet cmd is max 12 bytes.

There should be no functionality change resulting from this patch.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>