net/mlx5: Fix the size of modify QP mailbox

Add 16 reserved bytes at the end of mlx5_modify_qp_mbox_in to
match the hardware spec definition.

Fixes: e126ba97 ('mlx5: Add driver for Mellanox Connect-IB adapters')
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>