mtip32xx: fix dereference of stack garbage

We need to get the command payload from the request before
we attempt to dereference it.

Fixes: 4dda4735

 ("mtip32xx: add a status field to struct mtip_cmd")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>