IB/mlx5: Expose correct max_sge_rd limit

IB/mlx5: Expose correct max_sge_rd limit

mlx5 devices (Connect-IB, ConnectX-4, ConnectX-4-LX) has a limitation
where rdma read work queue entries cannot exceed 512 bytes.
A rdma_read wqe needs to fit in 512 bytes:
- wqe control segment (16 bytes)
- rdma segment (16 bytes)
- scatter elements (16 bytes each)

So max_sge_rd should be: (512 - 16 - 16) / 16 = 30.

Cc: [email protected]
Reported-by: Christoph Hellwig <[email protected]>
Tested-by: Christoph Hellwig <[email protected]>
Signed-off-by: Sagi Grimberg <[email protected]>
Signed-off-by: Leon Romanovsky <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>