IB/core: Set speed string to SDR for invalid active rates

Before commit f1b65df5 ("IB/mlx5: Add support for active_width and
active_speed in RoCE"), the mlx5_ib driver set default active_width and
active_speed to IB_WIDTH_4X and IB_SPEED_QDR.

Now, the active_width and active_speed are zeros if the RoCE port
is in DOWN state. The speed string should be set to " SDR" instead of
a blank string when active_speed is zero.

Signed-off-by: Honggang Li <honli@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>