net/mlx5: Expose port speed when possible

When port speed can't be reported based on ext_eth_proto_capability
or eth_proto_capability instead of reporting speed as unknown check
if the port's speed can be inferred based on the data_rate_oper field.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>