net: hns3: add querying fec statistics

FEC statistics can be used to check the transmission quality of links.
This patch implements the get_fec_stats callback of ethtool_ops to support
querying FEC statistics by command "ethtool -I --show-fec eth0".

Signed-off-by: Hao Lan <lanhao@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>