IB/mlx4: Don't return an invalid speed when a port is down

When the IB port is down, the active_speed value returned by the
MAD_IFC command is seven (7) which isn't among the defined IB speeds
in enum ib_port_speed, and this invalid speed value is passed up to
higher layers or applications who do port query.

Fix that by setting the speed to be SDR -- the lowest possible -- when
the port is down.

Signed-off-by: Roland Dreier <roland@purestorage.com>