scsi: 3w-9xxx: Use flexible array members to avoid struct padding

In preparation for removing the "#pragma pack(1)" from the driver, fix all
instances where a trailing array member could be replaced by a flexible
array member. Since a flexible array member has zero size, it introduces no
padding, whether or not the struct is packed.

Link: https://lore.kernel.org/r/20210427235915.39211-2-samuel@sholland.org


Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>