app/testpmd: add FEC command

app/testpmd: add FEC command

This commit adds testpmd capability to query and config FEC
function of device. This includes:
- show FEC capabilities, example:
	testpmd> show port 0 fec capabilities
- show FEC mode, example:
	testpmd> show port 0 fec_mode
- config FEC mode, example:
	testpmd> set port <port_id> fec_mode auto|off|rs|baser

	where:

	auto|off|rs|baser are four kinds of FEC mode which dev
	support according to MAC link speed.

Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Reviewed-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Chengwen Feng <fengchengwen@huawei.com>
Reviewed-by: Chengchang Tang <tangchengchang@huawei.com>