scsi: pm80xx: Avoid -Wflex-array-member-not-at-end warnings

Comment out unused field 'residual_count' in a couple of structures, and
with this, fix the following -Wflex-array-member-not-at-end warnings:

drivers/scsi/pm8001/pm8001_hwi.h:342:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/scsi/pm8001/pm80xx_hwi.h:561:32: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>