net: hns3: prevent double free in hns3_put_ring_config()

This patch adds a check for the hns3_put_ring_config() to prevent
double free, and for more readable, move the NULL assignment of
priv->ring_data into the hns3_put_ring_config().

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