net: hns3: fix access null pointer issue when set tx-buf-size as 0

When set tx-buf-size as 0 by ethtool, hns3_init_tx_spare_buffer()
will return directly and priv->ring->tx_spare->len is uninitialized,
then print function access priv->ring->tx_spare->len will cause
this issue.

When set tx-buf-size as 0 by ethtool, the print function will
print 0 directly and not access priv->ring->tx_spare->len.

Fixes: 2373b35c ("net: hns3: add log for setting tx spare buf size")
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>