net: hns3: add netdev reset check for hns3_set_tunable()

When pci device reset failed, it does uninit operation and priv->ring
is NULL, it causes accessing NULL pointer error.

Add netdev reset check for hns3_set_tunable() to fix it.

Fixes: 99f6b5fb

 ("net: hns3: use bounce buffer when rx page can not be reused")
Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>