RDMA/nes: Map MTU to IB_MTU_* and correctly report link state

Old query_port code reports static MTU and link state values.
Instead, map actual MTU to next largest IB_MTU_* constant and
correctly report link state.

Cc: Steve Wise <swise@opengridcomputing.com>
Reported-by: Jeff Squyres <jsquyres@cisco.com>
Signed-off-by: Chien Tung <chien.tin.tung@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>