net: hns3: add NULL pointer check for hns3_set/get_ringparam()

When pci devices init failed and haven't reinit, priv->ring is
NULL and hns3_set/get_ringparam() will access priv->ring. it
causes call trace.

So, add NULL pointer check for hns3_set/get_ringparam() to
avoid this situation.

Fixes: 5668abda

 ("net: hns3: add support for set_ringparam")
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>