net: hns3: fix for FEC configuration

The FEC capbility may be changed with port speed changes. Driver
needs to read the active FEC mode, and update FEC capability
when port speed changes.

Fixes: 7e6ec914 ("net: hns3: add support for FEC encoding control")
Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>