net: hns3: refine MAC pause statistics querying function

This patch refines the interface for querying MAC pause
statistics, and adds structure hns3_mac_stats to keep the
count of TX & RX.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Reviewed-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>