net: hns3: use capability flag to indicate FEC

Currently, the revision of the pci device is used to identify
whether FEC is supported, which is not good for maintainability
and compatibility. So use a capability flag to do that.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>