net: hns3: Fix for DEFAULT_DV when dev doesn't support DCB

When ae_dev doesn't support DCB, DEFAULT_DV must be set to
a lower value, otherwise the buffer allocation process will
fail.
This patch fix it by setting it to 30K bytes.

Fixes: 46a3df9f ("net: hns3: Add HNS3 Acceleration Engine & Compatibility Layer Support")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>