examples/l3fwd: force link speed

examples/l3fwd: force link speed

Currently, l3fwd starts in auto-negotiation mode, but it may fail to
link up when auto-negotiation is not supported. Therefore, it is
necessary to support starting with a specified speed for port.

Additionally, this patch does not support changing the duplex mode.
So speeds like 10M, 100M are not configurable using this method.

Signed-off-by: Dengdui Huang <huangdengdui@huawei.com>
Reviewed-by: Chengwen Feng <fengchengwen@huawei.com>