mlx4_core: Get the maximum message size from reported device capabilities

Get the maximum message size from the device capabilities returned
from the QUERY_DEV_CAP firmware command, rather than hard-coding 2 GB.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Signed-off-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>