net: hns3: fix the max tx size according to user manual

Currently the max tx size supported by the hw is calculated by
using the max BD num supported by the hw. According to the hw
user manual, the max tx size is fixed value for both non-TSO and
TSO skb.

This patch updates the max tx size according to the manual.

Fixes: 8ae10cfb("net: hns3: support tx-scatter-gather-fraglist feature")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>