net: hns: Modify GMAC init TX threshold value

This patch reduces GMAC TX threshold value to avoid gmac
hang-up with speed 100M/duplex half.

Signed-off-by: lipeng <lipeng321@huawei.com>
Signed-off-by: JinchuanTian <tianjinchuan1@huawei.com>
Reviewed-by: Yisen Zhuang <yisen.zhuang@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>