net/mlx4_core: Fix reading HCA max message size in mlx4_QUERY_DEV_CAP

Currently we parse max_msg_sz from the wrong offset in QUERY_DEV_CAP,
fix to use the right offset.

Fixes: 0b131561

 ('net/mlx4_en: Add Flow control statistics [..]')
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>