IB/mlx4: Utilize macro to calculate SQ spare size

The macro MLX4_IB_SQ_HEADROOM calculates the spare room needed to be
left. Use it instead of hard-coding the HW prefetch size.
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx4/qp.c

@@ -401,7 +401,7 @@ static int set_kernel_sq_size(struct mlx4_ib_dev *dev, struct ib_qp_cap *cap,
 	 * We need to leave 2 KB + 1 WR of headroom in the SQ to
 	 * allow HW to prefetch.
 	 */
-	qp->sq_spare_wqes = (2048 >> qp->sq.wqe_shift) + 1;
+	qp->sq_spare_wqes = MLX4_IB_SQ_HEADROOM(qp->sq.wqe_shift);
 	qp->sq.wqe_cnt = roundup_pow_of_two(cap->max_send_wr +
 					    qp->sq_spare_wqes);