RDMA/cxgb3: Set active_mtu in ib_port_attr

When running ibv_devinfo, the active_mtu returned is garbage.  This is
due to the field not being populated in the query_port function in the
driver.  The patch below populates the active_mtu field with a MTU of
2k.  It also zeros the struct, so that any new additions to it will
return 0.

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