RDMA/cxgb3: Set the max_mr_size device attribute correctly

cxgb3 only supports 4GB memory regions.  The lustre RDMA code uses
this attribute and currently has to code around our bad setting.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>