scsi: isci: Avoid -Wflex-array-member-not-at-end warning

scsi: isci: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration (which happens to be in a union, so
we're moving the entire union) to the end of the corresponding
structure. Notice that `struct ssp_response_iu` is a flexible structure,
this is a structure that contains a flexible-array member.

With these changes fix the following warning:

drivers/scsi/isci/task.h:92:11: warning: structure containing a flexible
array member is not at the end of another structure
[-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/aM09bpl1xj9KZSZl@kspp
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>