net: hns: use eth_get_headlen interface instead of hns_nic_get_headlen

Update hns to drop the hns_nic_get_headlen function in favour of
eth_get_headlen, and hence also removes now redundant hns_nic_get_headlen.

Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>