net: hns: add get_coalesce_range api for hns

This patch adds get_coalesce_range api for hns, it shows
range of coalesce usecs and frames that can be set on
this interface.

Signed-off-by: Daode Huang <huangdaode@hisilicon.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>