net/mlx5e: IPoIB, Show unknown speed instead of error

net/mlx5e: IPoIB, Show unknown speed instead of error

ethtool is returning an error for unknown speeds for the IPoIB interface:

$ ethtool ib0
netlink error: failed to retrieve link settings
netlink error: Invalid argument
netlink error: failed to retrieve link settings
netlink error: Invalid argument
Settings for ib0:
Link detected: no

After this change, ethtool will return success and show "unknown speed":

$ ethtool ib0
Settings for ib0:
Supported ports: [  ]
Supported link modes:   Not reported
Supported pause frame use: No
Supports auto-negotiation: No
Supported FEC modes: Not reported
Advertised link modes:  Not reported
Advertised pause frame use: No
Advertised auto-negotiation: No
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Full
Auto-negotiation: off
Port: Other
PHYAD: 0
Transceiver: internal
Link detected: no

Fixes: eb234ee9d541 ("net/mlx5e: IPoIB, Add support for get_link_ksettings in ethtool")
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>