RDMA/siw: Move tx_cpu ahead

We can reduce one cacheline for the usage of struct siw_qp.

Before,

	/* size: 1928, cachelines: 31, members: 38 */
	/* sum members: 1920, holes: 2, sum holes: 8 */
	/* paddings: 4, sum paddings: 13 */
	/* forced alignments: 3 */

after

	/* size: 1920, cachelines: 30, members: 38 */
	/* paddings: 4, sum paddings: 13 */
	/* forced alignments: 3 */

Link: https://lore.kernel.org/r/20231203092655.28102-2-guoqing.jiang@linux.dev


Acked-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>