IB/mlx4: Configure QPs' max message size based on real device capability

ConnectX returns the max message size it supports through the
QUERY_DEV_CAP firmware command.  When modifying a QP to RTR, the max
message size for the QP must be specified.  This value must not exceed
the value declared through QUERY_DEV_CAP.  The current code ignores
the max allowed size and unconditionally sets the value to 2^31.  This
patch sets all QPs to the max value allowed as returned from firmware.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>