net: hns: bug fix about setting coalsecs-usecs to 0

When set rx/tx coalesce usecs to 0, the interrupt coalesce will be
disabled, but there is a interrupt rate limit which set to 1us, it
will cause no interrupt occurs. This patch disable interrupt limit
when sets coalsecs usecs to 0, and restores it to 1 in other case.

Signed-off-by: Daode Huang <huangdaode@hisilicon.com>
Reviewed-by: Yisen Zhuang <yisen.zhuang@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>