net: hns3: add log for setting tx spare buf size

For the active tx spare buffer size maybe changed according
to the page size, so add log to notice it.

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>